Change language by clicking a button

By far the best (easiest) way is to use the locale filter (inside get_locale()).

First set up a quick function for retrieving a different language to use on the locale filter.

/**
 * A function returns with returns the user's selectd locale, if stored. 
 */
function wpse35622_get_new_locale($locale=false){
$new_locale = get_user_meta(get_current_user_id(), 'wpse_locale', true);
    if($new_locale)
        return $new_locale;

    return $locale;
}

I’ve left this as a global function, might be handy…? The rest of the plug-in I’ve wrapped inside a class.

It creates a drop-down of possible languages from which you can select.

Drawbacks

• Searches wp-content/languages for files of the form xx_xx.mo. If it’s not of that form (not all .mo files are) the plug-in won’t pick up on it!
• The drop down gives a list of the found locales to choose from, but these are displayed in the ‘xx_xx` format of locales, not a human friendly way – any ideas??
• Does $locale do anything other than language? If so it’s still possible to get the locale the same, but provide alternative translations. It’s a lot messier though…

The code

class SH_Pick_Lang{
    /**
     * A unique name for this plug-in
     */
    static $name="sh_pick_lang";

    /**
     * Hook the functions
     */
    public function __construct(){

        if( isset($_POST[self::$name]) ){
            add_action('locale',array($this,'update_user'));
        }
        add_filter( 'locale', 'wpse35622_get_new_locale',20 );
        add_action( 'wp_before_admin_bar_render', array( $this, 'admin_bar') );
    }

    /**
     * Update the user's option just in time! Only once mind...
     */
    function update_user($locale){
        $save_locale = $_POST[self::$name];
        update_user_meta(get_current_user_id(), 'wpse_locale', $save_locale);
        remove_filter(current_filter(),__FUNCTION__);
        return $locale;
    }

    /**
     * Add a really horrible looking dropdown menu that I'm sure Kaiser will make look fantastic.
     */
    function admin_bar(){
        global $wp_admin_bar;

        $wp_admin_bar->add_menu( array(
            'id'        => 'shlangpick',
            'title'     => $this->wpse_language_dropown(),
        ) );
    }

    /**
     * Generates a list of available locales.
     * Searches the wp-content/languages folder for files of the form xx_xx.mo
     * 
     * TODO Not all locales are of the form xx_xx.mo - we might miss some.
     * TODO Better way of gettin gthe wp-content/languages folder without a constant?
     */
    function wpse_language_dropown(){
         $name = self::$name;
         $locale = get_locale();
         $langs = get_available_languages();
         $langs[] = 'en_US';

         //For the labels (format_code_lang)
         require_once( ABSPATH . 'wp-admin/includes/ms.php');

         $html = "<form method='post'> ";
         $html .= "<select name="{$name}">";
         foreach($langs as $lang){
              $label = format_code_lang( $lang );
              $html .= "<option ".selected( $lang, $locale, false)." value="{$lang}">{$label}</option>";
        }
         $html .= "</select>";
         $html .= get_submit_button('Change Language','secondary', 'submit', true);
         $html .= "</form >";

         return $html;
    }

} // END Class

//Initiate...
$sh_pick_lang = new SH_Pick_Lang();