wpquestions.com, ID = 2901

<?php
/**
 * the below code goes into functions.php of the current theme
 * The first function registers WP nav menus for each locale on your site
 * It's possible to get the locales with the function get_available_languages()
 * or pass them manually as an array
 */
add_action( 'init', 'custom_register_nav_menus' );

function custom_register_nav_menus() {

    // get all languages in theme's languages folder (.mo files)
    // $available_locales = get_available_languages( STYLESHEETPATH . '/languages' );
    // or manually set available locales
    $available_locales = array ( 'en', 'zh' ) ;

    // register a nav menu for each of the locales
    foreach ( $available_locales as $locale ) {
        // the theme locations would be called "primary-en" and "primary-zh"
        register_nav_menu( "primary-{$locale}", "Main Menu ({$locale})" );
    }

}


/**
 * The below functions changes the WP's active locale based on the request uri
 * You must have pretty permalink enabled for this to work and the pages for
 * each language need to be under a master page with the slug of 'en' or 'zh'
 */
add_filter( 'locale', 'swpl_change_locale' );

function swpl_change_locale( $locale ) {

    global $swpl_customized_locale;

    // If we've already done this, return the value we previously found
    if ( ! empty( $swpl_customized_locale ) )
        return $swpl_customized_locale;

    // get all languages in theme's languages folder (.mo files)
    // $available_locales = get_available_languages( STYLESHEETPATH . '/languages' );
    // or manually set available locales
    $available_locales = array ( 'en', 'zh' ) ;

    // loop 'em
    foreach ( $available_locales as $lang ) {

        // if using get_available_languages(), the returned locale would be in the format 'en_US'; use this to convert it to 2-letter locale
        // $short_locale = strtolower ( substr( $lang, 0, 2 ) );

        // if request uri starts with one of the locales
        if ( strpos( $_SERVER[ 'REQUEST_URI' ], '/' . $short_locale ) === 0) {

            // assign locale
            $locale = $swpl_customized_locale = $lang;
            break;

        }

    }

    return $locale;

}



/**
 * Then call your menu like this in your template
 */
wp_nav_menu( array(
    'theme_location' => 'primary-' . get_locale(),
    )
);