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();
	    Zend_Registry::get('firebug')->info('Plugin Language');
	    $user = new Scores_Utilisateur();
	    if ( $user->isLog() ) {
    	    $lang = $request->getParam('lang', '');

    	    switch ($lang) {
    	        case "fr":
    	            $langLocale = 'fr';
    	            break;
    	        case "en":
    	            $langLocale = 'en';
    	            break;
    	        default:
    	            $langLocale = $session->lang !='' ? $session->lang : $user->getLang();
    	    }
	    } else {
	        $langLocale = 'fr';
	    }

	    $newLocale = new Zend_Locale();
	    $newLocale->setLocale($langLocale);
	    $registry->set('Zend_Locale', $newLocale);

	    $translate->setLocale($langLocale);
	    $this->setSessionLang($langLocale);

	    $registry->set('Zend_Translate', $translate);
	}

	/**
	 * Save new lang value into session
	 * @param string $lang
	 */
	protected function setSessionLang($lang)
	{
		$authStorage = Zend_Auth::getInstance()->getStorage();
		$authData = $authStorage->read();
		if($authData)
		{
			$authData->lang = $lang;
			$authStorage->write($authData);
		}
	}
}