Issue #1964482 by Eronarn: Fixed Non-canonical Symfony priority in LanguageRequestSubscriber.

core/lib/Drupal/Core/EventSubscriber/LanguageRequestSubscriber.php

@@ -55,7 +55,7 @@ public function onKernelRequestLanguage(GetResponseEvent $event) {
    *   An array of event listener definitions.
    */
   static function getSubscribedEvents() {
-    $events[KernelEvents::REQUEST][] = array('onKernelRequestLanguage', 300);
+    $events[KernelEvents::REQUEST][] = array('onKernelRequestLanguage', 255);
 
     return $events;
   }