Resolve #3394858 "Language redirects should be temporary"

Closes #3394858

src/EventSubscriber/RouteNormalizerRequestSubscriber.php

@@ -136,7 +136,20 @@ class RouteNormalizerRequestSubscriber implements EventSubscriberInterface {
       $original_uri = $request->getSchemeAndHttpHost() . $request->getRequestUri();
       $original_uri = urldecode($original_uri);
       if (urldecode($redirect_uri) != $original_uri) {
-        $response = new TrustedRedirectResponse($redirect_uri, $this->config->get('default_status_code'));
+        $status_code = $this->config->get('default_status_code');
+        $language_negotiation_config = \Drupal::config('language.negotiation')->get('url');
+        $current_langcode = \Drupal::languageManager()->getCurrentLanguage()->getId();
+        if ($language_negotiation_config['source'] == "path_prefix") {
+          $prefix = $language_negotiation_config['prefixes'][$current_langcode];
+          $redirect_relative_uri = $this->urlGenerator->generateFromRoute($route_name);
+          $prefixed_original_uri = rtrim('/' . $prefix . $request->getRequestUri(), '/');
+          if (urldecode($redirect_relative_uri) == $prefixed_original_uri) {
+            // Language redirections should be 302 as they are temporary.
+            $status_code = 302;
+          }
+        }
+
+        $response = new TrustedRedirectResponse($redirect_uri, $status_code);
         $response->headers->set('X-Drupal-Route-Normalizer', 1);
         $event->setResponse($response);
       }