Issue #2995655 by awm, Sam152, jedgar1mx, lpeabody, axel.rutz: Invalid...

Issue #2995655 by awm, Sam152, jedgar1mx, lpeabody, axel.rutz: Invalid translation language (und) specified. in Drupal\Core\Entity\ContentEntityBase when changing language of an entity

core/modules/content_translation/src/Plugin/Validation/Constraint/ContentTranslationSynchronizedFieldsConstraintValidator.php

@@ -191,6 +191,12 @@ protected function getOriginalEntity(ContentEntityInterface $entity) {
    *   The original entity translation object.
    */
   protected function getOriginalTranslation(ContentEntityInterface $entity, ContentEntityInterface $original) {
+    // If the language of the default translation is changing, the original
+    // translation will be the same as the original entity, but they won't
+    // necessarily have the same langcode.
+    if ($entity->isDefaultTranslation() && $original->language()->getId() !== $entity->language()->getId()) {
+      return $original;
+    }
     $langcode = $entity->language()->getId();
     if ($original->hasTranslation($langcode)) {
       $original_langcode = $langcode;

core/modules/content_translation/tests/src/Kernel/ContentTranslationFieldSyncRevisionTest.php

@@ -367,6 +367,16 @@ public function testFieldSynchronizationAndValidation() {
     $this->assertLatestRevisionFieldValues($entity_id, [26, 3, 3, 'Alt 3 EN', 'Alt 4 IT']);
   }
 
+  /**
+   * Test changing the default language of an entity.
+   */
+  public function testChangeDefaultLanguageNonTranslatableFieldsHidden() {
+    $this->setUntranslatableFieldWidgetsDisplay(FALSE);
+    $entity = $this->saveNewEntity();
+    $entity->langcode = 'it';
+    $this->assertCount(0, $entity->validate());
+  }
+
   /**
    * Sets untranslatable field widgets' display status.
    *