diff -u b/core/modules/language/lib/Drupal/language/Entity/Language.php b/core/modules/language/lib/Drupal/language/Entity/Language.php
--- b/core/modules/language/lib/Drupal/language/Entity/Language.php
+++ b/core/modules/language/lib/Drupal/language/Entity/Language.php
@@ -103,7 +103,7 @@
       // Update the config. Saving the configuration fires and event that causes
       // the container to be rebuilt.
       \Drupal::config('system.site')->set('langcode', $this->id())->save();
-      \Drupal::service('language.default')->set($this);
+      \Drupal::service('language.default')->set(\Drupal::languageManager()->getLanguage($this->id()));
     }
 
     $language_manager = \Drupal::languageManager();