reverted:
--- b/core/includes/update.inc
+++ a/core/includes/update.inc
@@ -207,10 +207,8 @@
     }
     variable_set('locale_translation_plurals', $plurals);
     variable_set('locale_translation_javascript', $javascript);
+    variable_set('locale_language_negotiation_url_prefixes', $prefixes);
+    variable_set('locale_language_negotiation_url_domains', $domains);
-    config('language.negotiation')
-      ->set('url.prefixes', $prefixes)
-      ->set('url.domains', $domains)
-      ->save();
 
     // Drop now unneeded columns.
     db_drop_field('languages', 'plurals');
diff -u b/core/modules/language/language.install b/core/modules/language/language.install
--- b/core/modules/language/language.install
+++ b/core/modules/language/language.install
@@ -46,8 +46,8 @@
   }
 
   config('language.negotiation')
-    ->delete('session.parameter')
-    ->delete('url.determination_part');
+    ->clear('session.parameter')
+    ->clear('url.determination_part');
 
   // Re-initialize the language system so successive calls to t() and other
   // functions will not expect languages to be present.
diff -u b/core/modules/locale/locale.install b/core/modules/locale/locale.install
--- b/core/modules/locale/locale.install
+++ b/core/modules/locale/locale.install
@@ -554,6 +554,20 @@
  * @ingroup config_upgrade
  */
 function locale_update_8008() {
+  $variable_name_map = array(
+    'locale_language_negotiation_url_part' => 'language_negotiation_url_part',
+    'locale_language_negotiation_url_domains' => 'language_negotiation_url_domains',
+    'locale_language_negotiation_url_prefixes' => 'language_negotiation_url_prefixes',
+    'locale_language_negotiation_session_param' => 'language_negotiation_session_param',
+  );
+  foreach ($variable_name_map as $deprecated_variable_name => $new_variable_name) {
+    // Check if this variable is stored in the db and if so rename it.
+    $value = update_variable_get($deprecated_variable_name);
+    if ($value !== NULL) {
+      update_variable_set($new_variable_name, $value);
+      update_variable_del($deprecated_variable_name);
+    }
+  }
   update_variables_to_config('language.negotiation', array(
     'locale_language_negotiation_session_param' => 'session.parameter',
     'language_negotiation_url_part' => 'url.determination_part',