Applied core patch against new forum module

Closes #3194394

src/Controller/ForumController.php

@@ -178,7 +178,7 @@ class ForumController extends ControllerBase {
    */
   public function forumIndex() {
     $vocabulary = $this->vocabularyStorage->load($this->config('forum.settings')->get('vocabulary'));
-    $index = $this->forumManager->getIndex();
+    $index = $this->forumManager->getIndex($this->languageManager()->getCurrentLanguage()->getId());
     $build = $this->build($index->forums, $index);
     if (empty($index->forums)) {
       // Root of empty forum.