Polylang sets the language in plugins_loaded

<?php
// filters posts and terms by language, rewrites links to include the language, etc...
// used only for frontend
class Polylang_Core extends Polylang_base {
    public $curlang; // current language

    private $default_locale;
    private $list_textdomains = array(); // all text domains
    private $labels; // post types and taxonomies labels to translate
    private $first_query = true;

    // options often needed
    private $page_for_posts;
    private $page_on_front;

    // used to cache results
    private $posts = array();
    private $translation_url = array();
    private $home_urls = array();

    function __construct() {
        parent::__construct();

        // init options often needed
        $this->page_for_posts = get_option('page_for_posts');
        $this->page_on_front = get_option('page_on_front');

        // if no language found, choose the preferred one
        add_filter('pll_get_current_language', array(&$this, 'pll_get_current_language'));

        // sets the language of comment
        add_action('pre_comment_on_post', array(&$this, 'pre_comment_on_post'));

        // text domain management
        if ($this->options['force_lang'] && get_option('permalink_structure')) {
            add_action('plugins_loaded', array(&$this, 'setup_theme'), 1);
            add_action('wp_loaded', array(&$this, 'add_language_filters'), 5); // after Polylang_Base::add_post_types_taxonomies
        }
        else {
            add_filter('override_load_textdomain', array(&$this, 'mofile'), 10, 3);
            add_filter('gettext', array(&$this, 'gettext'), 10, 3);
            add_filter('gettext_with_context', array(&$this, 'gettext_with_context'), 10, 4);
        }

        add_action('init', array(&$this, 'init'));
        foreach (array('wp', 'login_init', 'admin_init') as $filter) // admin_init for ajax thanks to g100g
            add_action($filter, array(&$this, 'load_textdomains'), 5); // priority 5 for post types and taxonomies registered in wp hook with default priority

        // filters the WordPress locale
        add_filter('locale', array(&$this, 'get_locale'));

        // filters posts according to the language
        add_filter('pre_get_posts', array(&$this, 'pre_get_posts'), 5);

        // filter sticky posts by current language
        add_filter('option_sticky_posts', array(&$this, 'option_sticky_posts'));

        // translates page for posts and page on front
        add_filter('option_page_for_posts', array(&$this, 'translate_page'));
        add_filter('option_page_on_front', array(&$this, 'translate_page'));
    }

    // set these filters and actions only once the current language has been defined
    function add_language_filters() {
        if (!$this->get_languages_list() || empty($this->curlang))
            return;

        // modifies the language information in rss feed (useful if WP < 3.4)
        add_filter('option_rss_language', array(&$this, 'option_rss_language'));

        // filters categories and post tags by language
        add_filter('terms_clauses', array(&$this, 'terms_clauses'), 10, 3);

        // meta in the html head section
        add_action('wp_head', array(&$this, 'wp_head'));

        // modifies the page link in case the front page is not in the default language
        add_filter('page_link', array(&$this, 'page_link'), 10, 2);

        // manages the redirection of the homepage
        add_filter('redirect_canonical', array(&$this, 'redirect_canonical'), 10, 2);

        // adds javascript at the end of the document
        if (!$GLOBALS['wp_rewrite']->using_permalinks() && (!defined('PLL_SEARCH_FORM_JS') || PLL_SEARCH_FORM_JS))
            add_action('wp_footer', array(&$this, 'wp_print_footer_scripts'));

        // adds the language information in the search form
        // low priority in case the search form is created using the same filter as described in http://codex.wordpress.org/Function_Reference/get_search_form
        add_filter('get_search_form', array(&$this, 'get_search_form'), 99);

        // adds the language information in admin bar search form
        remove_action('admin_bar_menu', 'wp_admin_bar_search_menu', 4);
        add_action('admin_bar_menu', array(&$this, 'admin_bar_search_menu'), 4);

        // filters the pages according to the current language in wp_list_pages
        add_filter('wp_list_pages_excludes', array(&$this, 'wp_list_pages_excludes'));

        // filters the comments according to the current language
        add_filter('comments_clauses', array(&$this, 'comments_clauses'), 10, 2);

        // rewrites archives, next and previous post links to filter them by language
        foreach (array('getarchives', 'get_previous_post', 'get_next_post') as $filter)
            foreach (array('_join', '_where') as $clause)
                add_filter($filter.$clause, array(&$this, 'posts'.$clause));

        // rewrites author and date links to filter them by language
        foreach (array('feed_link', 'author_link', 'post_type_archive_link', 'year_link', 'month_link', 'day_link') as $filter)
            add_filter($filter, array(&$this, 'archive_link'));

        $this->add_post_term_link_filters(); // these filters are in base as they may be used on admin side too

        // filters the nav menus according to the current language
        add_filter('theme_mod_nav_menu_locations', array(&$this, 'nav_menu_locations'));
        add_filter('wp_nav_menu_args', array(&$this, 'wp_nav_menu_args'));
        add_filter('wp_nav_menu_items', array(&$this, 'wp_nav_menu_items'), 10, 2);

        // filters the widgets according to the current language
        add_filter('widget_display_callback', array(&$this, 'widget_display_callback'), 10, 3);

        // strings translation (must be applied before WordPress applies its default formatting filters)
        foreach (array('widget_title', 'option_blogname', 'option_blogdescription', 'option_date_format', 'option_time_format') as $filter)
            add_filter($filter, 'pll__', 1);

        // translates biography
        add_filter('get_user_metadata', array(&$this,'get_user_metadata'), 10, 4);

        // modifies the home url
        if (!defined('PLL_FILTER_HOME_URL') || PLL_FILTER_HOME_URL)
            add_filter('home_url', array(&$this, 'home_url'), 10, 2);
    }

    // returns the language according to browser preference or the default language
    function get_preferred_language() {
        // check first is the user was already browsing this site
        if (isset($_COOKIE[PLL_COOKIE]))
            return $this->get_language($_COOKIE[PLL_COOKIE]);

        // compatibility with old cookie removed in 1.0
        if (isset($_COOKIE['wordpress_polylang']))
            return $this->get_language($_COOKIE['wordpress_polylang']);

        // sets the browsing language according to the browser preferences
        // code adapted from http://www.thefutureoftheweb.com/blog/use-accept-language-header
        if ($this->options['browser']) {
            $accept_langs = array();

            if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
                // break up string into pieces (languages and q factors)
                preg_match_all('/([a-z]{1,8}(-[a-z]{1,8})?)\s*(;\s*q\s*=\s*(1|0\.[0-9]+))?/i', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $lang_parse);

                $k = $lang_parse[1];
                $v = $lang_parse[4];

                if ($n = count($k)) {
                    // set default to 1 for any without q factor
                    foreach ($v as $key => $val)
                        if ($val === '') $v[$key] = 1;

                    // bubble sort (need a stable sort for Android, so can't use a PHP sort function)
                    if ($n > 1) {
                        for ($i = 2; $i <= $n; $i++)
                            for ($j = 0; $j <= $n-2; $j++)
                                if ( $v[$j] < $v[$j + 1]) {
                                    // swap values
                                    $temp = $v[$j];
                                    $v[$j] = $v[$j + 1];
                                    $v[$j + 1] = $temp;
                                    //swap keys
                                    $temp = $k[$j];
                                    $k[$j] = $k[$j + 1];
                                    $k[$j + 1] = $temp;
                                }
                    }
                    // NOTE: array_combine => PHP5
                    $accept_langs = array_combine($k,$v);
                }
            }

            // looks through sorted list and use first one that matches our language list
            $listlanguages = $this->get_languages_list(array('hide_empty' => true)); // hides languages with no post
            foreach (array_keys($accept_langs) as $accept_lang) {
                foreach ($listlanguages as $language) {
                    if (stripos($accept_lang, $language->slug) === 0 && !isset($pref_lang)) {
                        $pref_lang = $language;
                    }
                }
            }
        } // options['browser']

        // allow plugin to modify the preferred language (useful for example to have a different fallback than the default language)
        $slug = apply_filters('pll_preferred_language', isset($pref_lang) ? $pref_lang->slug : false);

        // return default if there is no preferences in the browser or preferences does not match our languages or it is requested not to use the browser preference
        return ($lang = $this->get_language($slug)) ? $lang : $this->get_language($this->options['default_lang']);
    }

    // returns the current language
    function get_current_language() {
        if ($this->curlang)
            return $this->curlang;

        // no language set for 404
        if (is_404() || current_filter() == 'login_init')
            return $this->get_preferred_language();

        if ($var = get_query_var('lang'))
            $lang = $this->get_language(reset(explode(',',$var))); // choose the first queried language

        // Ajax thanks to g100g
        elseif (isset($_REQUEST['pll_load_front']))
            $lang =  empty($_REQUEST['lang']) ? $this->get_preferred_language() : $this->get_language($_REQUEST['lang']);

        elseif ((is_single() || is_page() || (is_attachment() && $this->options['media_support'])) && ( ($var = get_queried_object_id()) || ($var = get_query_var('p')) || ($var = get_query_var('page_id')) || ($var = get_query_var('attachment_id')) ))
            $lang = $this->get_post_language($var);

        elseif (isset($this->taxonomies)) {
            foreach ($this->taxonomies as $taxonomy) {
                if ($var = get_query_var(get_taxonomy($taxonomy)->query_var))
                    $lang = $this->get_term_language($var, $taxonomy);
            }
        }
        // allows plugins to set the language
        return apply_filters('pll_get_current_language', isset($lang) ? $lang : false);
    }

    // if no language found, return the preferred one
    function pll_get_current_language($lang) {
        return !$lang ? $this->get_preferred_language() : $lang;
    }

    // sets the language of comment
    function pre_comment_on_post($post_id) {
        $this->curlang = $this->get_post_language($post_id);
        add_filter('page_link', array(&$this, 'page_link'), 10, 2); // useful when posting a comment on static front page in non default language
        $this->add_post_term_link_filters(); // useful to redirect to correct post comment url when adding the language to all url
    }

    // sets the language when it is always included in the url
    function setup_theme() {
        // this function was hooked to setup_theme with priority 5 (after Polylang::init)
        // has been moved to plugins_loaded, 1 due to WPSEO and to be consistent with WPML
        // but $wp_rewrite is not defined yet, so let register our taxonomy partially
        $using_permalinks = get_option('permalink_structure');
        $index = 'index.php'; // $wp_rewrite->index is hardcoded in wp-includes/rewrite.php
        register_taxonomy('language', null , array('label' => false, 'query_var'=>'lang', 'rewrite'=>false));

        if (!$languages_list = $this->get_languages_list())
            return;

        // special case for ajax request
        if (isset($_REQUEST['pll_load_front']))
            $this->curlang = empty($_REQUEST['lang']) ? $this->get_preferred_language() : $this->get_language($_REQUEST['lang']);

        // standard case
        else {
            foreach ($languages_list as $language)
                $languages[] = $language->slug;

            $root = $this->options['rewrite'] ? '' : 'language/';
            $languages = $using_permalinks ? '#\/'.$root.'('.implode('|', $languages).')\/#' : '#lang=('.implode('|', $languages).')#';
            preg_match($languages, trailingslashit($_SERVER['REQUEST_URI']), $matches);

            // home is resquested
            // some PHP setups turn requests for / into /index.php in REQUEST_URI
            // thanks to Gonçalo Peres for pointing out the issue with queries unknown to WP
            // http://wordpress.org/support/topic/plugin-polylang-language-homepage-redirection-problem-and-solution-but-incomplete?replies=4#post-2729566
            if (str_replace('www.', '', home_url('/')) == trailingslashit((is_ssl() ? 'https://' : 'http://').str_replace('www.', '', $_SERVER['HTTP_HOST']).str_replace(array($index, '?'.$_SERVER['QUERY_STRING']), array('', ''), $_SERVER['REQUEST_URI']))) {
                // take care to post & page preview http://wordpress.org/support/topic/static-frontpage-url-parameter-url-language-information
                if (isset($_GET['preview']) && ( (isset($_GET['p']) && $id = $_GET['p']) || (isset($_GET['page_id']) && $id = $_GET['page_id']) ))
                    $this->curlang = ($lg = $this->get_post_language($id)) ? $lg : $this->get_language($this->options['default_lang']);

                // take care to (unattached) attachments
                elseif (isset($_GET['attachment_id']) && $id = $_GET['attachment_id'])
                    $this->curlang = ($lg = $this->get_post_language($id)) ? $lg : $this->get_preferred_language();

                else
                    $this->home_requested();
            }

            // $matches[1] is the slug of the requested language
            elseif ($matches)
                $this->curlang = $this->get_language($matches[1]);

            // first test for wp-login, wp-signup, wp-activate
            // stripos for case insensitive file systems
            elseif (false === stripos($_SERVER['SCRIPT_NAME'], $index) || !$this->options['hide_default'])
                $this->curlang = $this->get_preferred_language();

            else
                $this->curlang = $this->get_language($this->options['default_lang']);

            if ($using_permalinks)
                add_action('wp', array(&$this, 'check_language_code_in_url')); // before Wordpress redirect_canonical
        }

        $GLOBALS['text_direction'] = get_metadata('term', $this->curlang->term_id, '_rtl', true) ? 'rtl' : 'ltr';
        $GLOBALS['l10n']['pll_string'] = $this->mo_import($this->curlang);
        do_action('pll_language_defined');
    }

    // save the default locale before we start any language manipulation
    function init() {
        $this->default_locale = get_locale();
    }

    // returns the locale based on current language
    function get_locale($locale) {
        return $this->curlang ? $this->curlang->description : $locale;
    }

    // modifies the language information in rss feed
    function option_rss_language($value) {
        return get_bloginfo_rss('language');
    }

    // saves all text domains in a table for later usage
    function mofile($bool, $domain, $mofile) {
        $this->list_textdomains[] = array ('mo' => $mofile, 'domain' => $domain);
        return true; // prevents WP loading text domains as we will load them all later
    }

    // saves post types and taxonomies labels for a later usage
    function gettext($translation, $text, $domain) {
        $this->labels[$text] =  array('domain' => $domain);
        return $translation;
    }

    // saves post types and taxonomies labels for a later usage
    function gettext_with_context($translation, $text, $context, $domain) {
        $this->labels[$text] =  array('domain' => $domain, 'context' => $context);
        return $translation;
    }

    // translates post types and taxonomies labels once the language is known
    function translate_labels($type) {
        foreach($type->labels as $key=>$label)
            if (is_string($label) && isset($this->labels[$label]))
                $type->labels->$key = isset($this->labels[$label]['context']) ?
                    _x($label, $this->labels[$label]['context'], $this->labels[$label]['domain']) :
                    __($label, $this->labels[$label]['domain']);
    }

    // NOTE: I believe there are two ways for a plugin to force the WP language
    // as done by xili_language: load text domains and reinitialize wp_locale with the action 'wp'
    // as done by qtranslate: define the locale with the action 'plugins_loaded', but in this case, the language must be specified in the url.
    function load_textdomains() {
        // our override_load_textdomain filter has done its job. let's remove it before calling load_textdomain
        remove_filter('override_load_textdomain', array(&$this, 'mofile'));
        remove_filter('gettext', array(&$this, 'gettext'), 10, 3);
        remove_filter('gettext_with_context', array(&$this, 'gettext_with_context'), 10, 4);

        // check there is at least one language defined and sets the current language
        if ($this->get_languages_list() && $this->curlang = $this->get_current_language()) {
            // since 1.0: suppress old cookie which conflicts with quick cache
            if (!headers_sent() && isset($_COOKIE['wordpress_polylang']))
                setcookie('wordpress_polylang', '', time() - 3600, COOKIEPATH, COOKIE_DOMAIN);

            // set a cookie to remember the language. check headers have not been sent to avoid ugly error
            // possibility to set PLL_COOKIE to false will disable cookie although it will break some functionalities
            if (!headers_sent() && PLL_COOKIE !== false && (!isset($_COOKIE[PLL_COOKIE]) || $_COOKIE[PLL_COOKIE] != $this->curlang->slug))
                setcookie(PLL_COOKIE, $this->curlang->slug, time() + 31536000 /* 1 year */, COOKIEPATH, COOKIE_DOMAIN);

            if (!($this->options['force_lang'] && $GLOBALS['wp_rewrite']->using_permalinks())) {
                // set all our language filters and actions
                $this->add_language_filters();

                // now we can load text domains with the right language
                $new_locale = get_locale();
                foreach ($this->list_textdomains as $textdomain) {
                    $mo = str_replace("{$this->default_locale}.mo", "{$new_locale}.mo", $textdomain['mo']);
                    // since WP3.5 themes may store languages files in /wp-content/languages/themes
                    $mo = file_exists($mo) ? $mo : WP_LANG_DIR . "/themes/{$textdomain['domain']}-{$new_locale}.mo";
                    load_textdomain($textdomain['domain'], $mo);
                }
                // reinitializes wp_locale for weekdays and months, as well as for text direction
                unset($GLOBALS['wp_locale']);
                $GLOBALS['wp_locale'] = new WP_Locale();
                $GLOBALS['wp_locale']->text_direction = get_metadata('term', $this->curlang->term_id, '_rtl', true) ? 'rtl' : 'ltr';

                // translate labels of post types and taxonomies
                foreach ($GLOBALS['wp_taxonomies'] as $tax)
                    $this->translate_labels($tax);
                foreach ($GLOBALS['wp_post_types'] as $pt)
                    $this->translate_labels($pt);

                // and finally load user defined strings
                $GLOBALS['l10n']['pll_string'] = $this->mo_import($this->curlang);
                do_action('pll_language_defined');
            }
        }

        else {
            // can't work so load the text domains with WordPress default language
            foreach ($this->list_textdomains as $textdomain)
                load_textdomain($textdomain['domain'], $textdomain['mo']);
        }

        // free memory
        unset($this->list_textdomains);
        unset($this->labels);
    }

    // special actions when home page is requested
    function home_requested($query = false) {
        $using_permalinks = get_option('permalink_structure'); // can't use $wp_rewrite if called early

        // need this filter to get the right url when adding language code to all urls
        if ($this->options['force_lang'] && $using_permalinks)
            add_filter('_get_page_link', array(&$this, 'post_link'), 10, 2);

        // FIXME cookie wordpress_polylang removed since 1.0
        // test referer in case PLL_COOKIE is set to false
        // thanks to Ov3rfly http://wordpress.org/support/topic/enhance-feature-when-front-page-is-visited-set-language-according-to-browser
        $this->curlang = $this->options['hide_default'] && ((isset($_SERVER['HTTP_REFERER']) && strpos($_SERVER['HTTP_REFERER'], $this->home) !== false)) ?
            $this->get_language($this->options['default_lang']) :
            $this->get_preferred_language(); // sets the language according to browser preference or default language

        if ($query)
            $this->_home_requested($query);
        else
            add_action('setup_theme', array(&$this, '_home_requested')); // delays actions which need $wp_query & $wp_rewrite
    }

    // sets the correct query var for home page
    // optionally redirects to the home page in the preferred language
    function _home_requested($query = false) {
        // we are already on the right page
        if ($this->options['default_lang'] == $this->curlang->slug && $this->options['hide_default']) {
            if ($this->page_on_front && $link_id = $this->get_post($this->page_on_front, $this->curlang))
                $query ? $query->set('page_id', $link_id) : set_query_var('page_id', $link_id);
            else
                $query ? $query->set('lang', $this->curlang->slug) : set_query_var('lang', $this->curlang->slug);
        }

        // redirect to the home page in the right language
        // test to avoid crash if get_home_url returns something wrong
        // FIXME why this happens? http://wordpress.org/support/topic/polylang-crashes-1
        // don't redirect if $_POST is not empty as it could break other plugins
        // don't forget the query string which may be added by plugins
        elseif (is_string($redirect = $this->get_home_url($this->curlang)) && empty($_POST)) {
            wp_redirect(empty($_SERVER['QUERY_STRING']) ? $redirect : $redirect . ($wp_rewrite->using_permalinks() ? '?' : '&') . $_SERVER['QUERY_STRING']);
            exit;
        }
    }

    // filters posts according to the language
    function pre_get_posts($query) {
        // don't make anything if no language has been defined yet
        // $this->post_types & $this->taxonomies are defined only once the action 'wp_loaded' has been fired
        // don't honor suppress_filters as it breaks adjacent_image_link when post_parent == 0
        if (!$this->get_languages_list() || !did_action('wp_loaded'))
            return;

        $qv = $query->query_vars;

        // do not filter if lang is set to an empty value
        if (isset($qv['lang']) && !$qv['lang'])
            return;

        // users may want to display content in a different language than the current one by setting it explicitely in the query
        if (!$this->first_query && $this->curlang && !empty($qv['lang']))
            return;

        $is_post_type = isset($qv['post_type']) && (
            in_array($qv['post_type'], $this->post_types) ||
            (is_array($qv['post_type']) && array_intersect($qv['post_type'], $this->post_types))
        );

        // don't filters post types not in our list
        if (isset($qv['post_type']) && !$is_post_type)
            return;

        global $wp_rewrite;
        $this->first_query = false;

        // special case for wp-signup.php & wp-activate.php
        // stripos for case insensitive file systems
        if (false === stripos($_SERVER['SCRIPT_NAME'], $wp_rewrite->index)) {
            $this->curlang = $this->get_preferred_language();
            return;
        }

        // homepage is requested, let's set the language
        // take care to avoid posts page for which is_home = 1
        if (!$this->curlang && empty($query->query) && (is_home() || (is_page() && $qv['page_id'] == $this->page_on_front)))
            $this->home_requested($query);

        // redirect the language page to the homepage
        if ($this->options['redirect_lang'] && is_tax('language') && $this->page_on_front && (count($query->query) == 1 || (is_paged() && count($query->query) == 2))) {
            $this->curlang = $this->get_language(get_query_var('lang'));
            if ($page_id = $this->get_post($this->page_on_front, $this->get_language(get_query_var('lang')))) {
                $query->set('page_id', $page_id);
                $query->is_singular = $query->is_page = true;
                $query->is_archive = $query->is_tax = false;
                unset($query->queried_object); // reset queried object
                return;
            }
            // else : the static front page is not translated
            // let's things as is and the list of posts in the current language will be displayed
        }

        // sets is_home on translated home page when it displays posts
        // is_home must be true on page 2, 3... too
        if (!$this->page_on_front && is_tax('language') && (count($query->query) == 1 || (is_paged() && count($query->query) == 2))) {
            $this->curlang = $this->get_language(get_query_var('lang')); // sets the language now otherwise it will be too late to filter sticky posts !
            $query->is_home = true;
            $query->is_archive = $query->is_tax = false;
        }

        // sets the language for posts page in case the front page displays a static page
        if ($this->page_for_posts) {
            // If permalinks are used, WordPress does set and use $query->queried_object_id and sets $query->query_vars['page_id'] to 0
            // and does set and use $query->query_vars['page_id'] if permalinks are not used :(
            if (!empty($qv['pagename']) && isset($query->queried_object_id))
                $page_id = $query->queried_object_id;

            elseif (isset($qv['page_id']))
                $page_id = $qv['page_id'];

            if (!empty($page_id) && $this->get_post($page_id, $this->get_post_language($this->page_for_posts)) == $this->page_for_posts) {
                $this->page_for_posts = $page_id;
                $this->curlang = $this->get_post_language($page_id);
                $query->set('lang', $this->curlang->slug);
                $query->is_singular = $query->is_page = false;
                $query->is_home = $query->is_posts_page = true;
            }
        }

        // FIXME to generalize as I probably forget things
        $is_archive = (count($query->query) == 1 && !empty($qv['paged'])) ||
            !empty($qv['m']) || !empty($qv['year']) || // need to test year due to post rewrite rule conflict when using date and name permalinks
            !empty($qv['author']) ||
            (isset($qv['post_type']) && is_post_type_archive() && $is_post_type);

        // sets 404 when the language is not set for archives needing the language in the url
        if (!$this->options['hide_default'] && !isset($qv['lang']) && !$wp_rewrite->using_permalinks() && $is_archive)
            $query->set_404();

        // sets the language in case we hide the default language
        if ($this->options['hide_default'] && !isset($qv['lang']) && ($is_archive || $query->is_search || (count($query->query) == 1 && !empty($qv['feed'])) ))
            $query->set('lang', $this->options['default_lang']);

        // allow filtering recent posts and secondary queries by the current language
        // take care not to break queries for non visible post types such as nav_menu_items, attachments...
        if (/*$query->is_home && */$this->curlang && (!isset($qv['post_type']) || $is_post_type ))
            $query->set('lang', $this->curlang->slug);

        // remove pages query when the language is set unless we do a search
        // FIXME is only search broken by this ?
        if (!empty($qv['lang']) && !isset($qv['post_type']) && !is_search())
            $query->set('post_type', 'post');

        // unset the is_archive flag for language pages to prevent loading the archive template
        // keep archive flag for comment feed otherwise the language filter does not work
        if (!empty($qv['lang']) && !is_comment_feed() &&
            !is_post_type_archive() && !is_date() && !is_author() && !is_category() && !is_tag() && !is_tax('post_format'))
            $query->is_archive = false;

        // unset the is_tax flag for authors pages and post types archives
        // FIXME Probably I should do this for other cases
        if (!empty($qv['lang']) && (is_author() || is_post_type_archive() || is_date() || is_search())) {
            $query->is_tax = false;
            unset($query->queried_object);
        }

        // sets a language for theme preview
        if (is_preview() && is_front_page()) {
            $this->curlang = $this->get_current_language();
            $query->set('lang', $this->curlang->slug);
        }

        // sets the language for an empty string search when hiding the code for default language
        // http://wordpress.org/support/topic/search-for-empty-string-in-default-language
        if (!$this->curlang && !get_query_var('lang') && $this->options['hide_default'] && isset($query->query['s']) && !$query->query['s'])
            $query->set('lang', $this->options['default_lang']);

        // to avoid conflict beetwen taxonomies
        if (isset($query->tax_query->queries))
            foreach ($query->tax_query->queries as $tax)
                if (in_array($tax['taxonomy'], $this->taxonomies))
                    unset($query->query_vars['lang']);
    }

    // filter sticky posts by current language
    function option_sticky_posts($posts) {
        if ($this->curlang && !empty($posts)) {
            foreach ($posts as $key=>$post_id) {
                if ($this->get_post_language($post_id)->term_id != $this->curlang->term_id)
                    unset($posts[$key]);
            }
        }
        return $posts;
    }

    // filters categories and post tags by language when needed
    function terms_clauses($clauses, $taxonomies, $args) {
        // does nothing except on taxonomies which are filterable
        foreach ($taxonomies as $tax) {
            if (!in_array($tax, $this->taxonomies))
                return $clauses;
        }

        // adds our clauses to filter by language
        return $this->_terms_clauses($clauses, isset($args['lang']) ? $args['lang'] : $this->curlang);
    }

    // meta in the html head section
    function wp_head() {
        // outputs references to translated pages (if exists) in the html head section
        foreach ($this->get_languages_list() as $language) {
            if ($language->slug != $this->curlang->slug && $url = $this->get_translation_url($language))
                printf("<link hreflang='%s' href='%s' rel='alternate' />\n", esc_attr($language->slug), esc_url($url));
        }
    }

    // modifies the page link in case the front page is not in the default language
    function page_link($link, $id) {
        if ($this->options['redirect_lang'] && $this->page_on_front && $lang = $this->get_post_language($id)) {
            if (!isset($this->posts[$lang->slug][$this->page_on_front]))
                $this->posts[$lang->slug][$this->page_on_front] = $this->get_post($this->page_on_front, $lang);
            if ($id == $this->posts[$lang->slug][$this->page_on_front])
                return $this->options['hide_default'] && $lang->slug == $this->options['default_lang'] ? trailingslashit($this->home) : get_term_link($lang, 'language');
        }

        if ($this->page_on_front && $this->options['hide_default']) {
            if (!isset($this->posts[$this->options['default_lang']][$this->page_on_front]))
                $this->posts[$this->options['default_lang']][$this->page_on_front] = $this->get_post($this->page_on_front, $this->options['default_lang']);
            if ($id == $this->posts[$this->options['default_lang']][$this->page_on_front])
                return trailingslashit($this->home);
        }

        return _get_page_link($id);
    }

    // manages canonical redirection of the homepage when using page on front
    function redirect_canonical($redirect_url, $requested_url) {
        global $wp_query;
        if (is_page() && !is_feed() && isset($wp_query->queried_object) && 'page' == get_option('show_on_front') && $wp_query->queried_object->ID == get_option('page_on_front'))
            return $this->options['redirect_lang'] ? $this->get_home_url() : false;
        return $redirect_url;
    }

    // redirects incoming links to the proper URL when adding the language code to all urls
    function check_language_code_in_url() {
        if (is_single() || is_page()) {
            global $post;
            if (isset($post->ID) && in_array($post->post_type, $this->post_types))
                $language = $this->get_post_language((int)$post->ID);
        }
        elseif (is_category() || is_tag() || is_tax()) {
            $obj = $GLOBALS['wp_query']->get_queried_object();
            if (in_array($obj->taxonomy, $this->taxonomies))
                $language = $this->get_term_language((int)$obj->term_id);
        }

        // the language is not correctly set so let's redirect to the correct url for this object
        if (isset($language) && $language->slug != $this->curlang->slug) {
            $root = $this->options['rewrite'] ? '/' : '/language/';
            foreach ($this->get_languages_list() as $lang)
                $languages[] = $root . $lang->slug;

            $requested_url  = (is_ssl() ? 'https://' : 'http://') . $_SERVER['HTTP_HOST'] . str_replace($languages, '', $_SERVER['REQUEST_URI']);
            $redirect_url = $this->add_language_to_link($requested_url, $language);
            wp_redirect($redirect_url, 301);
            exit;
        }
    }

    // adds some javascript workaround knowing it's not perfect...
    function wp_print_footer_scripts() {
        // modifies the search form since filtering get_search_form won't work if the template uses searchform.php or the search form is hardcoded
        // don't use directly e[0] just in case there is somewhere else an element named 's'
        // check before if the hidden input has not already been introduced by get_search_form (FIXME: is there a way to improve this ?
        // thanks to AndyDeGroo for improving the code for compatility with old browsers
        // http://wordpress.org/support/topic/development-of-polylang-version-08?replies=6#post-2645559

        $lang = esc_js($this->curlang->slug);
        $js = "//<![CDATA[
        e = document.getElementsByName('s');
        for (i = 0; i < e.length; i++) {
            if (e[i].tagName.toUpperCase() == 'INPUT') {
                s = e[i].parentNode.parentNode.children;
                l = 0;
                for (j = 0; j < s.length; j++) {
                    if (s[j].name == 'lang') {
                        l = 1;
                    }
                }
                if ( l == 0) {
                    var ih = document.createElement('input');
                    ih.type = 'hidden';
                    ih.name = 'lang';
                    ih.value = '$lang';
                    e[i].parentNode.appendChild(ih);
                }
            }
        }
        //]]>";
        echo "<script type='text/javascript'>" .$js. "</script>";
    }

    // adds the language information in the search form
    // does not work if searchform.php is used or if the search form is hardcoded in another template file
    function get_search_form($form) {
        if ($form)
            $form = $GLOBALS['wp_rewrite']->using_permalinks() ?
                str_replace(trailingslashit($this->home), $this->get_home_url($this->curlang, true), $form) :
                str_replace('</form>', '<input type="hidden" name="lang" value="'.esc_attr($this->curlang->slug).'" /></form>', $form);

        return $form;
    }

    // rewrites the admin bar search form to include the language
    function admin_bar_search_menu($wp_admin_bar) {
        global $wp_rewrite;

        $title = sprintf('
            <form action="%s" method="get" id="adminbarsearch">
                <input class="adminbar-input" name="s" id="adminbar-search" tabindex="10" type="text" value="" maxlength="150" />
                <input type="submit" class="adminbar-button" value="%s"/>
                %s
            </form>',
            $wp_rewrite->using_permalinks() ? $this->get_home_url($this->curlang, true) : esc_url($this->home),
            __('Search'),
            $wp_rewrite->using_permalinks() ? '' : sprintf('<input type="hidden" name="lang" value="%s" />', esc_attr($this->curlang->slug))
        );

        $wp_admin_bar->add_menu(array(
            'parent' => 'top-secondary',
            'id'     => 'search',
            'title'  => $title,
            'meta'   => array('class' => 'admin-bar-search', 'tabindex' => -1)
        ));
    }

    // excludes pages which are not in the current language for wp_list_pages
    // useful for the pages widget
    function wp_list_pages_excludes($pages) {
        return array_merge($pages, $this->exclude_pages($this->curlang->term_id));
    }

    // filters the comments according to the current language mainly for the recent comments widget
    function comments_clauses($clauses, $query) {
        return $this->_comments_clauses($clauses, isset($query->query_vars['lang']) ? $query->query_vars['lang'] : $this->curlang);
    }

    // modifies the sql request for wp_get_archives an get_adjacent_post to filter by the current language
    function posts_join($sql) {
        global $wpdb;
        return $sql . " INNER JOIN $wpdb->term_relationships AS pll_tr ON pll_tr.object_id = ID";
    }

    // modifies the sql request for wp_get_archives and get_adjacent_post to filter by the current language
    function posts_where($sql) {
        global $wpdb;
        preg_match("#post_type = '([^']+)'#", $sql, $matches);  // find the queried post type
        return !empty($matches[1]) && in_array($matches[1], $this->post_types) ? $sql . $wpdb->prepare(" AND pll_tr.term_taxonomy_id IN (%s)", $this->curlang->term_taxonomy_id) : $sql;
    }

    // modifies the author and date links to add the language parameter (as well as feed link)
    function archive_link($link) {
        return $this->add_language_to_link($link, $this->curlang);
    }

    // returns the url of the translation (if exists) of the current page
    function get_translation_url($language) {
        if (isset($this->translation_url[$language->slug]))
            return $this->translation_url[$language->slug];

        global $wp_query, $wp_rewrite;
        $qv = $wp_query->query;
        $hide = $this->options['default_lang'] == $language->slug && $this->options['hide_default'];

        // post and attachment
        if (is_single() && ($this->options['media_support'] || !is_attachment()) && $id = $this->get_post($wp_query->queried_object_id, $language))
            $url = get_permalink($id);

        // page for posts
        elseif (get_option('show_on_front') == 'page' && !empty($wp_query->queried_object_id) && $wp_query->queried_object_id == $this->page_for_posts && ($id = $this->get_post($this->page_for_posts, $language)))
            $url = get_permalink($id);

        elseif (is_page() && $id = $this->get_post($wp_query->queried_object_id, $language))
            $url = $hide && $id == $this->get_post($this->page_on_front, $language) ? $this->home : get_page_link($id);

        elseif (!is_tax('post_format') && !is_tax('language') && (is_category() || is_tag() || is_tax()) ) {
            $term = get_queried_object();
            $lang = $this->get_term_language($term->term_id);

            if (!$lang || $language->slug == $lang->slug)
                $url = get_term_link($term, $term->taxonomy); // self link
            elseif ($link_id = $this->get_translation('term', $term->term_id, $language))
                $url = get_term_link(get_term($link_id, $term->taxonomy), $term->taxonomy);
        }

        // don't test if there are existing translations before creating the url as it would be very expensive in sql queries
        elseif (is_archive()) {
            if ($wp_rewrite->using_permalinks()) {
                $filters = array('author_link', 'post_type_archive_link', 'year_link', 'month_link', 'day_link');

                // prevents filtering links by current language
                remove_filter('term_link', array(&$this, 'term_link')); // for post format
                foreach ($filters as $filter)
                    remove_filter($filter, array(&$this, 'archive_link'));

                if (is_author())
                    $url = $this->add_language_to_link(get_author_posts_url(0, $qv['author_name']), $language);

                elseif (is_year())
                    $url = $this->add_language_to_link(get_year_link($qv['year']), $language);

                elseif (is_month())
                    $url = $this->add_language_to_link(get_month_link($qv['year'], $qv['monthnum']), $language);

                elseif (is_day())
                    $url = $this->add_language_to_link(get_day_link($qv['year'], $qv['monthnum'], $qv['day']), $language);

                elseif (is_post_type_archive())
                    $url = $this->add_language_to_link(get_post_type_archive_link($qv['post_type']), $language);

                elseif (is_tax('post_format'))
                    $url = $this->add_language_to_link(get_post_format_link($qv['post_format']), $language);

                // put our language filters again
                add_filter('term_link', array(&$this, 'term_link'), 10, 3);
                foreach ($filters as $filter)
                    add_filter($filter, array(&$this, 'archive_link'));
            }
            else {
                $url = $hide ? remove_query_arg('lang') : add_query_arg('lang', $language->slug);
                $url = remove_query_arg('paged', $url);
            }
        }

        elseif (is_search()) {
            if ($wp_rewrite->using_permalinks())
                $url = $this->add_language_to_link($this->home.'/?'.$_SERVER['QUERY_STRING'], $language);
            else {
                $url = add_query_arg('lang', $language->slug);
                $url = remove_query_arg('paged', $url);
            }
        }

        elseif (is_home() || is_tax('language') )
            $url = $this->get_home_url($language);

        return $this->translation_url[$language->slug] = (isset($url) && !is_wp_error($url) ? $url : null);
    }

    // filters the nav menus according to the current language when called from get_nav_menu_locations()
    // mainly for Artisteer generated themes and themes which test has_nav_menu
    function nav_menu_locations($menus) {
        $menu_lang = get_option('polylang_nav_menus');
        foreach(array_keys(get_registered_nav_menus()) as $location) {
            if (isset($menu_lang[$location][$this->curlang->slug]))
                $menus[$location] = $menu_lang[$location][$this->curlang->slug];
        }
        return $menus;
    }

    // filters the nav menus according to the current language when called from wp_nav_menus
    function wp_nav_menu_args($args) {
        $menu_lang = get_option('polylang_nav_menus');

/* FIXME breaks more than it solves
        // attempt to find a theme location if the theme registered one but calls wp_nav_menu only with a harcoded menu
        if (!$args['theme_location'] && $args['menu']) {
            $menu = wp_get_nav_menu_object($args['menu']);
            foreach ($this->get_languages_list() as $language)
                foreach ($menu_lang as $location => $arr)
                    if (isset($arr[$language->slug]) && $arr[$language->slug] == $menu->term_id)
                        $args['theme_location'] = $location;
        }

        // attempt to find a theme location if the theme registered one (only one!) but calls wp_nav_menu without it
        if (!$args['theme_location'] && ($menus = get_registered_nav_menus()) && count($menus) == 1)
            $args['theme_location'] = key($menus);
*/
        if ($args['theme_location'] && isset($menu_lang[$args['theme_location']][$this->curlang->slug]))
            $args['menu'] = $menu_lang[$args['theme_location']][$this->curlang->slug];

        return $args;
    }

    // adds the language switcher at the end of the menu
    function wp_nav_menu_items($items, $args) {
        $menu_lang = get_option('polylang_nav_menus');
        return isset($args->theme_location) && isset($menu_lang[$args->theme_location]['switcher']) && $menu_lang[$args->theme_location]['switcher'] ?
            $items . $this->the_languages(array_merge($menu_lang[$args->theme_location], array('menu' => 1, 'echo' => 0))) : $items;
    }

    // filters the widgets according to the current language
    function widget_display_callback($instance, $widget, $args) {
        $widget_lang = get_option('polylang_widgets');
        // don't display if a language filter is set and this is not the current one
        return !empty($widget_lang[$widget->id]) && $widget_lang[$widget->id] != $this->curlang->slug ? false : $instance;
    }

    // translates biography
    function get_user_metadata($null, $id, $meta_key, $single) {
        return $meta_key == 'description' ? get_user_meta($id, 'description_'.$this->curlang->slug, true) : $null;
    }

    // translates page for posts and page on front
    function translate_page($v) {
        // returns the current page if there is no translation to avoid ugly notices
        // the fonction is often called so let's store the result
        return isset($this->curlang) && $v && (isset($this->posts[$v]) || $this->posts[$v] = $this->get_post($v, $this->curlang)) ? $this->posts[$v] : $v;
    }

    // filters the home url to get the right language
    function home_url($url, $path) {
        if (!(did_action('template_redirect') || did_action('login_init')) || rtrim($url,'/') != $this->home)
            return $url;

        $theme = get_theme_root();
        $is_get_search_form = false;

        // FIXME can I decrease the size of the array to improve speed?
        foreach (array_reverse(debug_backtrace(/*!DEBUG_BACKTRACE_PROVIDE_OBJECT|DEBUG_BACKTRACE_IGNORE_ARGS*/)) as $trace) {
            // search form
            if (isset($trace['file']) && strpos($trace['file'], 'searchform.php'))
                return $GLOBALS['wp_rewrite']->using_permalinks() ? $this->get_home_url($this->curlang, true) : $url;

            // don't interfere with get_search_form filter which I prefer to use when possible
            if ($trace['function'] == 'get_search_form')
                $is_get_search_form = true;

            if ($trace['function'] == 'wp_nav_menu' || $trace['function'] == 'login_footer' ||
                // direct call from the theme
                ( !$is_get_search_form && isset($trace['file']) && strpos($trace['file'], $theme) !== false && in_array($trace['function'], array('home_url', 'get_home_url', 'bloginfo', 'get_bloginfo')) ))
                // remove trailing slash if there is none in requested url
                return empty($path) ? rtrim($this->get_home_url($this->curlang), '/') : $this->get_home_url($this->curlang);
        }

        return $url;
    }

    // returns the home url in the right language
    function get_home_url($language = '', $is_search = false) {
        if ($language == '')
            $language = $this->curlang;

        if (isset($this->home_urls[$language->slug][$is_search]))
            return $this->home_urls[$language->slug][$is_search];

        if ($this->options['default_lang'] == $language->slug && $this->options['hide_default'])
            return $this->home_urls[$language->slug][$is_search] = trailingslashit($this->home);

        // a static page is used as front page : /!\ don't use get_page_link to avoid infinite loop
        // don't use this for search form
        if (!$is_search && $this->page_on_front && $id = $this->get_post($this->page_on_front, $language))
            return $this->home_urls[$language->slug][$is_search] = $this->page_link('', $id);

        $link = get_term_link($language, 'language');
        // add a trailing slash as done by WP on homepage (otherwise could break the search form when the permalink structure does not include one)
        // only for pretty permalinks
        return $this->home_urls[$language->slug][$is_search] = $GLOBALS['wp_rewrite']->using_permalinks() ? trailingslashit($link) : $link;
    }

    // displays (or returns) the language switcher
    function the_languages($args = '') {
        $defaults = array(
            'dropdown'               => 0, // display as list and not as dropdown
            'echo'                   => 1, // echoes the list
            'hide_if_empty'          => 1, // hides languages with no posts (or pages)
            'menu'                   => 0, // not for nav menu
            'show_flags'             => 0, // don't show flags
            'show_names'             => 1, // show language names
            'display_names_as'       => 'name', // valid options are slug and name
            'force_home'             => 0, // tries to find a translation
            'hide_if_no_translation' => 0, // don't hide the link if there is no translation
            'hide_current'           => 0, // don't hide current language
            'post_id'                => null, // if not null, link to translations of post defined by post_id
        );
        extract(wp_parse_args($args, $defaults));

        if ($dropdown)
            $output = $this->dropdown_languages(array('hide_empty' => $hide_if_empty, 'selected' => $this->curlang->slug));

        else {
            $output = '';

            foreach ($this->get_languages_list(array('hide_empty' => $hide_if_empty)) as $language) {
                // hide current language
                if ($this->curlang->term_id == $language->term_id && $hide_current)
                    continue;

                $url = $post_id !== null && ($tr_id = $this->get_post($post_id, $language)) ? get_permalink($tr_id) :
                    $post_id === null && !$force_home ? $this->get_translation_url($language) : null;

                $class = isset($url) ? '' : 'no-translation ';

                $url = apply_filters('pll_the_language_link', $url, $language->slug, $language->description);

                // hide if no translation exists
                if (!isset($url) && $hide_if_no_translation)
                    continue;

                $url = isset($url) ? $url : $this->get_home_url($language); // if the page is not translated, link to the home page

                $class .= sprintf('lang-item lang-item-%d lang-item-%s', esc_attr($language->term_id), esc_attr($language->slug));
                $class .= $language->term_id == $this->curlang->term_id ? ' current-lang' : '';
                $class .= $menu ? ' menu-item' : '';

                $flag = $show_flags ? $this->get_flag($language) : '';
                $name = $show_names || !$show_flags ? esc_html($display_names_as == 'slug' ? $language->slug : $language->name) : '';

                $output .= sprintf("<li class='%s'><a hreflang='%s' href='%s'>%s</a></li>\n",
                    $class,
                    esc_attr($language->slug),
                    esc_url($url),
                    $show_flags && $show_names ? $flag.'&nbsp;'.$name : $flag.$name
                );
            }
        }

        $output = apply_filters('pll_the_languages', $output, $args);

        if(!$echo)
            return $output;
        echo $output;
    }
}