Backend\Core\Engine\Url::setLocale PHP Method

    private function setLocale()
    {
        $default = $this->get('fork.settings')->get('Core', 'default_interface_language');
        $locale = $default;
        $possibleLocale = array_keys(BackendLanguage::getInterfaceLanguages());
        // is the user authenticated
        if (Authentication::getUser()->isAuthenticated()) {
            $locale = Authentication::getUser()->getSetting('interface_language', $default);
        } elseif (CommonCookie::exists('interface_language')) {
            // no authenticated user, but available from a cookie
            $locale = CommonCookie::get('interface_language');
        }
        // validate if the requested locale is possible
        if (!in_array($locale, $possibleLocale)) {
            $locale = $default;
        }
        BackendLanguage::setLocale($locale);
    }