diff -u b/core/modules/language/config/language.mappings.yml b/core/modules/language/config/language.mappings.yml
--- b/core/modules/language/config/language.mappings.yml
+++ b/core/modules/language/config/language.mappings.yml
@@ -2,8 +2,8 @@
 # these defaults handles the most common cases.
-zh-tw: zh-hant-tw # Taiwan Chinese in traditional script
-zh-hk: zh-hant-hk # Hong Kong Chinese in traditional script
-zh-mo: zh-hant-mo # Macao Chinese in traditional script
+zh-tw: zh-hant # Taiwan Chinese in traditional script
+zh-hk: zh-hant # Hong Kong Chinese in traditional script
+zh-mo: zh-hant # Macao Chinese in traditional script
 zh-cht: zh-hant # traditional Chinese
-zh-cn: zh-hans-cn # PRC Mainland Chinese in simplified script
-zh-sg: zh-hans-sg # Singapore Chinese in simplified script
+zh-cn: zh-hans # PRC Mainland Chinese in simplified script
+zh-sg: zh-hans # Singapore Chinese in simplified script
 zh-chs: zh-hans # simplified Chinese
diff -u b/core/modules/language/language.admin.inc b/core/modules/language/language.admin.inc
--- b/core/modules/language/language.admin.inc
+++ b/core/modules/language/language.admin.inc
@@ -820,6 +820,30 @@
 function language_negotiation_configure_browser_form($form, &$form_state) {
   $form = array();
 
+  // Initialize a language list to the ones available, including English.
+  drupal_static_reset('language_list');
+  $languages = language_list();
+
+  $existing_languages = array();
+  foreach ($languages as $langcode => $language) {
+    $existing_languages[$langcode] = $language->name;
+  }
+
+  // If we have no languages available, present the list of predefined languages
+  // only. If we do have already added languages, set up two option groups with
+  // the list of existing and then predefined languages.
+  if (empty($existing_languages)) {
+    $language_options = language_admin_predefined_list();
+    $default = key($language_options);
+  }
+  else {
+    $default = key($existing_languages);
+    $language_options = array(
+      t('Existing languages') => $existing_languages,
+      t('Languages not yet added') => language_admin_predefined_list()
+    );
+  }
+
   $form['mappings'] = array(
     '#type' => 'container',
     '#tree' => TRUE,
@@ -836,9 +860,9 @@
         '#required' => TRUE,
       ),
       'drupal_langcode' => array(
-        '#type' => 'textfield',
+        '#type' => 'select',
+        '#options' => $language_options,
         '#default_value' => $drupal_langcode,
-        '#size' => 20,
         '#required' => TRUE,
       ),
     );
diff -u b/core/modules/language/lib/Drupal/language/Tests/LanguageBrowserDetectionUnitTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageBrowserDetectionUnitTest.php
--- b/core/modules/language/lib/Drupal/language/Tests/LanguageBrowserDetectionUnitTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageBrowserDetectionUnitTest.php
@@ -196,11 +196,11 @@
     // Add a new custom mapping.
     $edit = array(
       'mappings[_new][browser_langcode]' => 'xx',
-      'mappings[_new][drupal_langcode]' => 'yy',
+      'mappings[_new][drupal_langcode]' => 'en',
     );
     $this->drupalPost('admin/config/regional/language/detection/browser', $edit, t('Save configuration'));
     $this->assertField('edit-mappings-xx-browser-langcode', 'xx', 'Browser language code found.');
-    $this->assertField('edit-mappings-xx-drupal-langcode', 'yy', 'Drupal language code found.');
+    $this->assertField('edit-mappings-xx-drupal-langcode', 'en', 'Drupal language code found.');
 
     // Add the same custom mapping again.
     $this->drupalPost('admin/config/regional/language/detection/browser', $edit, t('Save configuration'));
@@ -217,7 +217,7 @@
     // Add a new custom mapping with only a Drupal language code.
     $edit = array(
       'mappings[_new][browser_langcode]' => '',
-      'mappings[_new][drupal_langcode]' => 'yyyy',
+      'mappings[_new][drupal_langcode]' => 'en',
     );
     $this->drupalPost('admin/config/regional/language/detection/browser', $edit, t('Save configuration'));
     $this->assertText(t('Both browser and Drupal language codes must be provided.'), 'Error message displayed.');
@@ -226,17 +226,17 @@
     $edit = array(
       'mappings[xx][browser_langcode]' => 'zh-sg',
-      'mappings[xx][drupal_langcode]' => 'yy',
+      'mappings[xx][drupal_langcode]' => 'en',
     );
     $this->drupalPost('admin/config/regional/language/detection/browser', $edit, t('Save configuration'));
     $this->assertText(t('Browser language codes must be unique.'), 'Error message displayed.');
 
-    // Change Drupal language code of our custom mapping to zh-hans-sg.
+    // Change Drupal language code of our custom mapping to zh-hans.
     $edit = array(
       'mappings[xx][browser_langcode]' => 'xx',
-      'mappings[xx][drupal_langcode]' => 'zh-hans-sg',
+      'mappings[xx][drupal_langcode]' => 'zh-hans',
     );
     $this->drupalPost('admin/config/regional/language/detection/browser', $edit, t('Save configuration'));
     $this->assertField('edit-mappings-xx-browser-langcode', 'xx', 'Browser language code found.');
-    $this->assertField('edit-mappings-xx-drupal-langcode', 'zh-hans-sg', 'Drupal language code found.');
+    $this->assertField('edit-mappings-xx-drupal-langcode', 'zh-hans', 'Drupal language code found.');
   }
 }