diff --git a/core/modules/content_translation/src/ContentTranslationHandler.php b/core/modules/content_translation/src/ContentTranslationHandler.php
index 3979fff..8b3b998 100644
--- a/core/modules/content_translation/src/ContentTranslationHandler.php
+++ b/core/modules/content_translation/src/ContentTranslationHandler.php
@@ -545,7 +545,10 @@ protected function addTranslatabilityClue(&$element) {
     // check is because some elements have a #title attribute even though it is
     // not rendered, e.g. field containers.
     if (isset($element['#type']) && isset($fapi_title_elements[$element['#type']]) && isset($element['#title'])) {
-      $element['#title'] .= $suffix;
+      $element['#title'] = array(
+        '#plain_text' => $element['#title'],
+        '#suffix' => $suffix,
+      );
     }
     // If the current element does not have a (valid) title, try child elements.
     elseif ($children = Element::children($element)) {