diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/LanguageSelectWidget.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/LanguageSelectWidget.php
index 05f2aa1..567276c 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/LanguageSelectWidget.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/LanguageSelectWidget.php
@@ -11,7 +11,6 @@
 use Drupal\Core\Field\WidgetBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Language\LanguageInterface;
-use Drupal\Core\TypedData\TranslatableInterface;
 
 /**
  * Plugin implementation of the 'Language' widget.
diff --git a/core/modules/content_translation/src/ContentTranslationHandler.php b/core/modules/content_translation/src/ContentTranslationHandler.php
index 3bd1b38..1aae420 100644
--- a/core/modules/content_translation/src/ContentTranslationHandler.php
+++ b/core/modules/content_translation/src/ContentTranslationHandler.php
@@ -144,27 +144,33 @@ public function entityFormAlter(array &$form, FormStateInterface $form_state, En
       }
     }
 
-    // Disable languages for existing translations, so it is not possible to
-    // switch this node to some language which is already in the translation
-    // set.
-    $langcode_key = $this->entityType->getKey('langcode') ?: 'langcode';
-    $language_widget = NULL;
-    if (isset($form[$langcode_key]['widget'][0]['value']['#type'])) {
-      $language_widget = &$form[$langcode_key]['widget'][0]['value'];
-      if (!$is_translation && $language_widget['#type'] == 'language_select' && $has_translations) {
+    // Locate the language widget.
+    $language_field = $this->entityType->getKey('langcode') ?: 'langcode';
+    if (isset($form[$language_field])) {
+      $language_widget = &$form[$language_field];
+    }
+
+    // If we are editing the source entity, force the list of languages so that
+    // it is not possible to switch to a language for which a translation
+    // already exists. Note that this will only work if the widget is structured
+    // like \Drupal\Core\Field\Plugin\Field\FieldWidget\LanguageSelectWidget.
+    if (isset($language_widget['widget'][0]['value']) && !$is_translation && $has_translations) {
+      $language_select = &$language_widget['widget'][0]['value'];
+      if ($language_select['#type'] == 'language_select') {
         $options = array();
         foreach (language_list(LanguageInterface::STATE_CONFIGURABLE) as $language) {
+          // Show the current language, and the languages for which no
+          // translation already exists.
           if (empty($translations[$language->getId()]) || $language->getId() == $entity_langcode) {
-//            $form['langcode']['#options'][$language->getId()] = $language->getName();
-              $options[$language->getId()] = $language->getName();
+            $options[$language->getId()] = $language->getName();
           }
         }
-        $language_widget['#options'] = $options;
+        $language_select['#options'] = $options;
       }
     }
 
     if ($is_translation) {
-      if ($language_widget) {
+      if (isset($language_widget)) {
         $language_widget['#access'] = FALSE;
       }
 
@@ -276,8 +282,8 @@ public function entityFormAlter(array &$form, FormStateInterface $form_state, En
         '#default_value' => $new_translation ? '' : format_date($date, 'custom', 'Y-m-d H:i:s O'),
       );
 
-      if ($language_widget) {
-        $form[$langcode_key]['#multilingual'] = TRUE;
+      if (isset($language_widget)) {
+        $language_widget['#multilingual'] = TRUE;
       }
 
       $form['#process'][] = array($this, 'entityFormSharedElements');