Backend\Core\Engine\Ajax::validateLogin PHP Method

    private function validateLogin()
    {
        // check if the user is logged on, if not he shouldn't load any JS-file
        if (!Authentication::isLoggedIn()) {
            throw new Exception('Not logged in.');
        }
        // set interface language
        BackendLanguage::setLocale(Authentication::getUser()->getSetting('interface_language'));
    }