diff --git a/core/modules/ckeditor/src/Tests/CKEditorTest.php b/core/modules/ckeditor/src/Tests/CKEditorTest.php
index 91fc410..fedffe8 100644
--- a/core/modules/ckeditor/src/Tests/CKEditorTest.php
+++ b/core/modules/ckeditor/src/Tests/CKEditorTest.php
@@ -392,10 +392,14 @@ function testJSTranslation() {
 
   /**
    * Assert that CKEditor picks the expected language when French is default.
+   *
+   * @param string $langcode
+   *   Language code to assert for. Defaults to French. That is the default
+   *   language set in this assertion.
    */
   protected function assertCKEditorLanguage($langcode = 'fr') {
     ConfigurableLanguage::createFromLangcode('fr')->save();
-    \Drupal::config('system.site')->set('langcode', 'fr');
+    \Drupal::config('system.site')->set('langcode', 'fr')->save();
     $editor = entity_load('editor', 'filtered_html');
     $settings = $this->ckeditor->getJSSettings($editor);
     $this->assertEqual($settings['language'], $langcode);