diff -u b/core/modules/language/language.module b/core/modules/language/language.module
--- b/core/modules/language/language.module
+++ b/core/modules/language/language.module
@@ -688,13 +688,4 @@
   $language = $languages[$form_state['values']['site_default_language']];
-
-  \Drupal::config('system.site')->set('langcode', $language->id)->save();
-  \Drupal::service('language.default')->set($language);
-
-  \Drupal::languageManager()->reset();
-
-  // Update URL Prefixes for all languages after the new default language is
-  // propagated and the LanguageManagerInterface::getLanguages() cache is
-  // flushed.
-  language_negotiation_url_prefixes_update();
-
+  $language->default = TRUE;
+  language_save($language);
 }