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
@@ -1,7 +1,9 @@
-zh-tw: zh-hant-tw
-zh-hk: zh-hant-hk
-zh-mo: zh-hant-mo
-zh-cht: zh-hant
-zh-cn: zh-hans-cn
-zh-sg: zh-hans-sg
-zh-chs: zh-hans
+# Most browser do not send abbreviated languages codes not standard
+# supported by Drupal, 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-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-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
@@ -816,7 +816,8 @@
 
   $form['info'] = array(
     '#type' => 'markup',
-    '#markup' => t('You can add custom mappings to map browser languages to Drupal languages.'),
+    '#markup' => t('You can add custom mappings to map browser languages to Drupal languages.') . '<br />'
+      . t('You can use this to support browser language codes not standard supported by Drupal.'),
   );
 
   $form['mappings'] = array(
@@ -825,18 +826,20 @@
     '#theme' => 'language_negotiation_configure_browser_form_table',
   );
 
-  $mappings = language_get_mappings();
+  $mappings = language_get_browser_drupal_langcode_mappings();
   foreach ($mappings as $browser_langcode => $drupal_langcode) {
     $form['mappings'][$browser_langcode] = array(
       'browser_langcode' => array(
         '#type' => 'textfield',
         '#default_value' => $browser_langcode,
         '#size' => 20,
+        '#required' => TRUE,
       ),
       'drupal_langcode' => array(
         '#type' => 'textfield',
         '#default_value' => $drupal_langcode,
         '#size' => 20,
+        '#required' => TRUE,
       ),
     );
   }
@@ -919,7 +922,7 @@
   foreach ($mappings as $key => $data) {
     if ($key == '_new') {
       if (!empty($data['browser_langcode']) && !empty($data['drupal_langcode'])) {
-        // Make sure browser_langcode is unique
+        // Make sure browser_langcode is unique.
         if (array_key_exists($data['browser_langcode'], $unique_values)) {
           form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes must be unique.');
         }
@@ -937,7 +940,7 @@
     }
     else {
       if (!empty($data['browser_langcode']) && !empty($data['drupal_langcode'])) {
-        // Make sure browser_langcode is unique
+        // Make sure browser_langcode is unique.
         if (array_key_exists($data['browser_langcode'], $unique_values)) {
           form_set_error('mappings][' . $key . '][browser_langcode', 'Browser language codes must be unique.');
         }
@@ -968,7 +971,7 @@
 function language_negotiation_configure_browser_form_submit($form, &$form_state) {
   $mappings = $form_state['mappings'];
   if (!empty($mappings)) {
-    language_set_mappings($mappings);
+    language_set_browser_drupal_langcode_mappings($mappings);
   }
 }
 
@@ -989,10 +992,10 @@
  */
 function language_negotiation_configure_browser_delete_form_submit($form, &$form_state) {
   $browser_langcode = $form_state['browser_langcode'];
-  $mappings = language_get_mappings();
+  $mappings = language_get_browser_drupal_langcode_mappings();
   if (array_key_exists($browser_langcode, $mappings)) {
     unset($mappings[$browser_langcode]);
-    language_set_mappings($mappings);
+    language_set_browser_drupal_langcode_mappings($mappings);
   }
   $form_state['redirect'] = 'admin/config/regional/language/detection/browser';
 }
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
@@ -618,9 +618,13 @@
 }
 
 /**
- * Returns language mappings.
+ * Returns language mappings between browser and Drupal language codes.
+ *
+ * @return array
+ *   An array containing browser language codes as keys with corresponding
+ *   Drupal language codes as values.
  */
-function language_get_mappings() {
+function language_get_browser_drupal_langcode_mappings() {
   $config = config('language.mappings');
   if ($config->isNew()) {
     config_install_default_config('module', 'language');
@@ -630,9 +634,13 @@
 }
 
 /**
- * Stores language mappings.
+ * Stores language mappings between browser and Drupal language codes.
+ *
+ * @param array $mappings
+ *   An array containing browser language codes as keys with corresponding
+ *   Drupal language codes as values.
  */
-function language_set_mappings($mappings) {
+function language_set_browser_drupal_langcode_mappings($mappings) {
   $config = config('language.mappings');
   $config->setData($mappings);
   $config->save();
diff -u b/core/modules/language/language.negotiation.inc b/core/modules/language/language.negotiation.inc
--- b/core/modules/language/language.negotiation.inc
+++ b/core/modules/language/language.negotiation.inc
@@ -58,6 +58,15 @@
 /**
  * Identify language from the Accept-language HTTP header we got.
  *
+ * The algorithm works as follows:
+ * - map browser language codes to Drupal language codes.
+ * - order all browser language codes by qvalue from high to low.
+ * - add generic browser language codes if they aren't already specified
+ *   but with a slightly lower qvalue.
+ * - find the most specific Drupal language code with the highest qvalue.
+ * - if 2 or more languages are having the same qvalue, respect the order of
+ *   them inside the $languages array.
+ *
  * We perform browser accept-language parsing only if page cache is disabled,
  * otherwise we would cache a user-specific preference.
  *
@@ -83,7 +92,7 @@
   if (preg_match_all('@(?<=[, ]|^)([a-zA-Z-]+|\*)(?:;q=([0-9.]+))?(?:$|\s*,\s*)@', trim($_SERVER['HTTP_ACCEPT_LANGUAGE']), $matches, PREG_SET_ORDER)) {
     // Load custom mappings to support browsers that are sending non standard
     // language codes.
-    $mappings = language_get_mappings();
+    $mappings = language_get_browser_drupal_langcode_mappings();
     foreach ($matches as $match) {
       if ($mappings) {
         $langcode = strtolower($match[1]);
@@ -111,7 +120,7 @@
   // http://blogs.msdn.com/b/ie/archive/2006/10/17/accept-language-header-for-internet-explorer-7.aspx
   asort($browser_langcodes);
   foreach ($browser_langcodes as $langcode => $qvalue) {
-    // For chinese languages the generic tag is either zh-hans or zh-hant, so we
+    // For Chinese languages the generic tag is either zh-hans or zh-hant, so we
     // need to handle this separately.
     $generic_tag = '';
     if (strlen($langcode) > 7 && (substr($langcode, 0, 7) == 'zh-hant' || substr($langcode, 0, 7) == 'zh-hans')) {
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
@@ -27,8 +27,15 @@
 
   /**
    * Unit tests for the language_from_browser() function.
+   *
+   * @see language_from_browser().
    */
   function testLanguageFromBrowser() {
+    // The order of the languages is only important if the browser language
+    // codes are having the same qvalue, otherwise the one with the highest
+    // qvalue is prefered. The automatically generated generic tags are always
+    // having a lower qvalue.
+
     $languages = array(
       // In our test case, 'en' has priority over 'en-US'.
       'en' => new Language(array(
@@ -153,6 +160,10 @@
     }
   }
 
+  /**
+   * Tests for adding, editing and deleting mappings between browser language
+   * codes and Drupal language codes.
+   */
   function testUIBrowserLanguageMappings() {
     // User to manage languages.
     $admin_user = $this->drupalCreateUser(array('administer languages', 'access administration pages'));
@@ -170,10 +181,10 @@
     // 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,
+    $message = t('Are you sure you want to delete @browser_langcode?', array(
+      '@browser_langcode' => $browser_langcode,
     ));
-    $this->assertText($message, 'Question found.');
+    $this->assertRaw($message, 'Question found.');
 
     // Confirm the delete.
     $edit = array();