CODE
    public static function switchLanguage($context = null)
    {
        if(!$context) {
            $context = Context::getContext();
        }

        // Install call the dispatcher and so the switchLanguage
        // Stop this method by checking the cookie
        if(!isset($context->cookie)) {
            return;
        }

        $configuration_id_lang = (int)Configuration::get('PS_LANG_DEFAULT');
        if(($iso = Tools::getValue('isolang'))
            && Validate::isLanguageIsoCode($iso)
            && ($id_lang = (int)Language::getIdByIso($iso))
        ) {
            $_GET['id_lang'] = $id_lang;
        }
        else {
            $_GET['id_lang'] = $configuration_id_lang;
        }

        // Update language only if new id is different from old id
        // or if default language changed
        $cookie_id_lang = $context->cookie->id_lang;
        if((($id_lang = (int)Tools::getValue('id_lang'))
            && Validate::isUnsignedInt($id_lang)
            && $cookie_id_lang != (int)$id_lang)
            || (($id_lang == $configuration_id_lang)
                && Validate::isUnsignedInt($id_lang)
                && $id_lang != $cookie_id_lang)
        ) {
            $context->cookie->id_lang = $id_lang;
            $language = new Language($id_lang);
            if(Validate::isLoadedObject($language) 
                && $language->active 
                && $language->visible
            ) {
                $context->language = $language;
            }

            $params = $_GET;
            if(Configuration::get('PS_REWRITING_SETTINGS')
                || !Language::isMultiLanguageActivated()
            ) {
                unset($params['id_lang']);
            }
        }
    }