Contao\CoreBundle\EventListener\LocaleListener::getLocale PHP Method

    private function getLocale(Request $request)
    {
        if (null !== $request->attributes->get('_locale')) {
            return $this->formatLocaleId($request->attributes->get('_locale'));
        }
        if (null !== ($session = $request->getSession()) && $session->has('_locale')) {
            return $session->get('_locale');
        }
        return $request->getPreferredLanguage($this->availableLocales);
    }