extranet/library/Application/Controller/Plugin/Language.php

<?php
class Application_Controller_Plugin_Language extends Zend_Controller_Plugin_Abstract 
{
	public function preDispatch(Zend_Controller_Request_Abstract $request) 
	{
	    $registry = Zend_Registry::getInstance();
	    $session = new Zend_Session_Namespace('Zend_Auth');
	    $translate = $registry->get('Zend_Translate');
	    $currLocale = $translate->getLocale();
	    
	    $user = new Scores_Utilisateur();
	   	
	    $lang = $request->getParam('lang', '');
	    
	    switch ($lang) {
	        case "fr":
	            $langLocale = 'fr';
	            break;
	        case "en":
	            $langLocale = 'en';
	            break;
	        default:
	            $langLocale = $session->lang !='' ? $session->lang : $user->getLang();
	    }
	
	    $newLocale = new Zend_Locale();
	    $newLocale->setLocale($langLocale);
	    $registry->set('Zend_Locale', $newLocale);
	    

	    $session->lang = $langLocale;
	    
	    $translate->setLocale($langLocale);
	    
	    $registry->set('Zend_Translate', $translate);
	}
}