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
@@ -841,7 +841,7 @@
     );
   }
 
-  // Add empty row
+  // Add empty row.
   $form['mappings']['_new'] = array(
     'browser_langcode' => array(
       '#type' => 'textfield',
@@ -931,6 +931,9 @@
         }
         $unique_values[$data['browser_langcode']] = $data['drupal_langcode'];
       }
+      elseif (empty($data['browser_langcode']) xor empty($data['drupal_langcode'])) {
+        form_set_error('mappings][' . $key . '][drupal_langcode', 'Both browser and Drupal language codes must be provided.');
+      }
     }
     else {
       if (!empty($data['browser_langcode']) && !empty($data['drupal_langcode'])) {
@@ -970,7 +973,7 @@
 }
 
 /**
- * Delete a language mapping form.
+ * Delete a browser language negotiation mapping.
  */
 function language_negotiation_configure_browser_delete_form($form, &$form_state, $browser_langcode) {
   $form_state['browser_langcode'] = $browser_langcode;
@@ -982,7 +985,7 @@
 }
 
 /**
- * Delete a language mapping submit.
+ * Delete a browser language negotiation mapping.
  */
 function language_negotiation_configure_browser_delete_form_submit($form, &$form_state) {
   $browser_langcode = $form_state['browser_langcode'];
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
@@ -639 +639 @@
-}
\ No newline at end of file
+}
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
@@ -28,7 +28,7 @@
   /**
    * Unit tests for the language_from_browser() function.
    */
-  function testLanguageFromBrowser() {
+  function testLanguamergeFromBrowser() {
     $languages = array(
       // In our test case, 'en' has priority over 'en-US'.
       'en' => new Language(array(
@@ -154,2 +154,78 @@
   }
+
+  function testUIBrowserLanguageMappings() {
+    // User to manage languages.
+    $admin_user = $this->drupalCreateUser(array('administer languages', 'access administration pages'));
+    $this->drupalLogin($admin_user);
+
+    // Check that the configure link exists.
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->assertLinkByHref('admin/config/regional/language/detection/browser');
+
+    // Check that defaults are loaded from language.mappings.yml.
+    $this->drupalGet('admin/config/regional/language/detection/browser');
+    $this->assertField('edit-mappings-zh-cn-browser-langcode', 'zh-cn', 'Chinese browser language code found.');
+    $this->assertField('edit-mappings-zh-cn-drupal-langcode', 'zh-hans-cn', 'Chinese Drupal language code found.');
+
+    // Delete zh-cn language code.
+    $browser_langcode = 'zh-cn';
+    $this->drupalGet('admin/config/regional/language/detection/browser/delete/' . $browser_langcode);
+    $message = t('Are you sure you want to delete !browser_langcode?', array(
+      '!browser_langcode' => $browser_langcode,
+    ));
+    $this->assertText($message, 'Question found.');
+
+    // Confirm the delete.
+    $edit = array();
+    $this->drupalPost('admin/config/regional/language/detection/browser/delete/' . $browser_langcode, $edit, t('Confirm'));
+
+    // Check that ch-zn no longer exists.
+    $this->assertNoField('edit-mappings-zh-cn-browser-langcode', 'Chinese browser language code no longer exists.');
+
+    // Add a new custom mapping.
+    $edit = array(
+      'mappings[_new][browser_langcode]' => 'xx',
+      'mappings[_new][drupal_langcode]' => 'yy',
+    );
+    $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.');
+
+    // Add the same custom mapping again.
+    $this->drupalPost('admin/config/regional/language/detection/browser', $edit, t('Save configuration'));
+    $this->assertText('Browser language codes must be unique.', 'Error message displayed.');
+
+    // Add a new custom mapping with only a browser language code.
+    $edit = array(
+      'mappings[_new][browser_langcode]' => 'xxxx',
+      'mappings[_new][drupal_langcode]' => '',
+    );
+    $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.');
+
+    // Add a new custom mapping with only a Drupal language code.
+    $edit = array(
+      'mappings[_new][browser_langcode]' => '',
+      'mappings[_new][drupal_langcode]' => 'yyyy',
+    );
+    $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.');
+
+    // Change browser language code of our custom mapping to zh-sg.
+    $edit = array(
+      'mappings[xx][browser_langcode]' => 'zh-sg',
+      'mappings[xx][drupal_langcode]' => 'yy',
+    );
+    $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.
+    $edit = array(
+      'mappings[xx][browser_langcode]' => 'xx',
+      'mappings[xx][drupal_langcode]' => 'zh-hans-sg',
+    );
+    $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.');
+  }
 }