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
@@ -869,17 +869,25 @@
   }
 
   // Add empty row.
-  $form['mappings']['_new'] = array(
-    'browser_langcode' => array(
-      '#type' => 'textfield',
-      '#default_value' => '',
-      '#size' => 20,
-    ),
-    'drupal_langcode' => array(
-      '#type' => 'select',
-      '#options' => $language_options,
-      '#default_value' => '',
-    ),
+  $form['new_mapping'] = array(
+    '#type' => 'fieldset',
+    '#title' => t('Add a new mapping'),
+    '#collapsible' => TRUE,
+    '#collapsed' => TRUE,
+    '#tree' => TRUE,
+  );
+  $form['new_mapping']['browser_langcode'] = array(
+    '#type' => 'textfield',
+    '#title' => t('Browser language code'),
+    '#description' => t('Use language codes as <a href="@w3ctags">defined by the W3C</a> for interoperability. <em>Examples: "en", "en-gb" and "zh-hant".</em>', array('@w3ctags' => 'http://www.w3.org/International/articles/language-tags/')),
+    '#default_value' => '',
+    '#size' => 20,
+  );
+  $form['new_mapping']['drupal_langcode'] = array(
+    '#type' => 'select',
+    '#title' => t('Drupal langauge'),
+    '#options' => $language_options,
+    '#default_value' => '',
   );
 
   $form_state['redirect'] = 'admin/config/regional/language/detection';
@@ -914,13 +922,8 @@
     $row = array();
     $row[] = drupal_render($form[$key]['browser_langcode']);
     $row[] = drupal_render($form[$key]['drupal_langcode']);
+    $row[] = l(t('Delete'), 'admin/config/regional/language/detection/browser/delete/' . $key, $link_attributes);
 
-    if ($key != '_new') {
-      $row[] = l(t('Delete'), 'admin/config/regional/language/detection/browser/delete/' . $key, $link_attributes);
-    }
-    else {
-      $row[] = '';
-    }
     $rows[] = array(
       'data' => $row,
     );
@@ -941,42 +944,38 @@
  * Browser language negotiation form validation.
  */
 function language_negotiation_configure_browser_form_validate($form, &$form_state) {
-  $mappings = $form_state['values']['mappings'];
+  // Array to check if all browser language codes are unique.
   $unique_values = array();
-  foreach ($mappings as $key => $data) {
-    if ($key == '_new') {
-      if (!empty($data['browser_langcode']) && !empty($data['drupal_langcode'])) {
-        // Make sure browser_langcode is unique.
-        if (array_key_exists($data['browser_langcode'], $unique_values)) {
-          form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes must be unique.');
-        }
-        elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
-          form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes can only contain lowercase letters and a hyphen(-).');
-        }
-        $unique_values[$data['browser_langcode']] = $data['drupal_langcode'];
+
+  // Check all mappings.
+  $mappings = array();
+  if (isset($form_state['values']['mappings'])) {
+    $mappings = $form_state['values']['mappings'];
+    foreach ($mappings as $key => $data) {
+      // Make sure browser_langcode is unique.
+      if (array_key_exists($data['browser_langcode'], $unique_values)) {
+        form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes must be unique.');
       }
-      elseif (empty($data['browser_langcode']) xor empty($data['drupal_langcode'])) {
-        form_set_error('mappings][' . $key . '][drupal_langcode', 'Both browser language code and Drupal language must be provided.');
+      elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
+        form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes can only contain lowercase letters and a hyphen(-).');
       }
+      $unique_values[$data['browser_langcode']] = $data['drupal_langcode'];
     }
-    else {
-      if (!empty($data['browser_langcode']) && !empty($data['drupal_langcode'])) {
-        // Make sure browser_langcode is unique.
-        if (array_key_exists($data['browser_langcode'], $unique_values)) {
-          form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes must be unique.');
-        }
-        elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
-          form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes can only contain lowercase letters and a hyphen(-).');
-        }
-        $unique_values[$data['browser_langcode']] = $data['drupal_langcode'];
-      }
-      else {
-        if (empty($data['browser_langcode'])) {
-          form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language code cannot be blank.');
-        }
-      }
+  }
+
+  // Check new mapping.
+  $data = $form_state['values']['new_mapping'];
+  if (!empty($data['browser_langcode'])) {
+    // Make sure browser_langcode is unique.
+    if (array_key_exists($data['browser_langcode'], $unique_values)) {
+      form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes must be unique.');
+    }
+    elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
+      form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes can only contain lowercase letters and a hyphen(-).');
     }
+    $unique_values[$data['browser_langcode']] = $data['drupal_langcode'];
   }
+
   $form_state['mappings'] = $unique_values;
 }
 
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
@@ -195,8 +195,8 @@
 
     // Add a new custom mapping.
     $edit = array(
-      'mappings[_new][browser_langcode]' => 'xx',
-      'mappings[_new][drupal_langcode]' => 'en',
+      'new_mapping[browser_langcode]' => 'xx',
+      'new_mapping[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.');
@@ -206,14 +206,6 @@
     $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 Drupal language code.
-    $edit = array(
-      'mappings[_new][browser_langcode]' => '',
-      'mappings[_new][drupal_langcode]' => 'en',
-    );
-    $this->drupalPost('admin/config/regional/language/detection/browser', $edit, t('Save configuration'));
-    $this->assertText(t('Both browser language code and Drupal language 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',