function my_theme_localized($locale)
{
    if (isset($_GET['lang'])){
        return sanitize_key($_GET['lang']);
    }

    return $locale;
}
add_filter('locale', 'my_theme_localized');

function my_theme_setup(){
    load_theme_textdomain('theme', TRANSLATION_URL);
}
add_action('after_setup_theme', 'my_theme_setup');