Symfony\Component\HttpFoundation\Request::getPreferredLanguage PHP Method

    public function getPreferredLanguage(array $locales = null)
    {
        $preferredLanguages = $this->getLanguages();

        if (empty($locales)) {
            return isset($preferredLanguages[0]) ? $preferredLanguages[0] : null;
        }

        if (!$preferredLanguages) {
            return $locales[0];
        }

        $extendedPreferredLanguages = array();
        foreach ($preferredLanguages as $language) {
            $extendedPreferredLanguages[] = $language;
            if (false !== $position = strpos($language, '_')) {
                $superLanguage = substr($language, 0, $position);
                if (!in_array($superLanguage, $preferredLanguages)) {
                    $extendedPreferredLanguages[] = $superLanguage;
                }
            }
        }

        $preferredLanguages = array_values(array_intersect($extendedPreferredLanguages, $locales));

        return isset($preferredLanguages[0]) ? $preferredLanguages[0] : $locales[0];
    }

 /**
  * Chooses a language for a request
  *
  * @return $response or null
  **/
 public function switchLocaleForRequest(Request $request)
 {
     $session = $request->getSession();
     $parts = explode('.', $request->getHost());
     if (count($parts) === 3) {
         $locale = $parts[0];
         if ($locale == $session->getLocale()) {
             return;
         }
         if ($this->manager->isAvailable($locale)) {
             $session->setLocale($locale);
             $preferred = $request->getPreferredLanguage($this->manager->getAvailableLanguageCodes());
             if ($preferred != $locale) {
                 $session->setFlash('locale_change_adjust', $preferred);
             } else {
                 $session->setFlash('locale_change_contribute', $locale);
             }
             return;
         }
         $host = $parts[1] . '.' . $parts[2];
     } else {
         $host = $parts[0] . '.' . $parts[1];
     }
     $locale = $request->getPreferredLanguage($this->manager->getAvailableLanguageCodes());
     $url = sprintf('http://%s.%s%s', $locale, $host, $request->getRequestUri());
     $response = new RedirectResponse($url);
     $preferredLanguage = $this->getRequestPreferredLanguage($request);
     if ($preferredLanguage && $locale != $preferredLanguage) {
         $allLanguageCodes = array_keys($this->manager->getLanguages());
         if (in_array($preferredLanguage, $allLanguageCodes)) {
             $session->setFlash('locale_missing', $preferredLanguage);
         }
     }
     return $response;
 }