code: public function onKernelRequest(GetResponseEvent $event): void
      {
          $token = $this->tokenStorage->getToken();
  
          if (!$token instanceof TokenInterface) {
              return;
          }
  
          $user = $token->getUser();
  
          if (!$user instanceof BackendUser || !$user->language) {
              return;
          }
  
          $request = $event->getRequest();
          $request->setLocale($user->language);
  
          $this->translator->setLocale($user->language);
  
          // Deprecated since Contao 4.0, to be removed in Contao 5.0
          $GLOBALS['TL_LANGUAGE'] = str_replace('_', '-', $user->language);
      }
docs: Sets the default locale based on the user language.