diff --git a/includes/language.inc b/includes/language.inc
index d0ea83113371ea7517548774a6d790d696a64eb1..ea63948d980d7b012fd118293f6dcd7d598243cf 100644
--- a/includes/language.inc
+++ b/includes/language.inc
@@ -2,7 +2,9 @@
 
 /**
  * @file
- * Multiple language handling functionality.
+ * Language Negotiation API.
+ *
+ * @see http://drupal.org/node/1497272
  */
 
 /**
@@ -11,7 +13,96 @@
 define('LANGUAGE_NEGOTIATION_DEFAULT', 'language-default');
 
 /**
- * Return all the defined language types.
+ * @defgroup language_negotiation Language Negotiation API functionality
+ * @{
+ * Functions to customize the language types and the negotiation process.
+ *
+ * The language negotiation API is based on two major concepts:
+ * - Language types: types of translatable data (the types of data that a user
+ *   can view or request).
+ * - Language negotiation providers: functions for determining which language to
+ *   use to present a particular piece of data to the user.
+ * Both language types and language negotiation providers are customizable.
+ *
+ * Drupal defines three built-in language types:
+ * - Interface language: The page's main language, used to present translated
+ *   user interface elements such as titles, labels, help text, and messages.
+ * - Content language: The language used to present content that is available
+ *   in more than one language (see
+ *   @link field_language Field Language API @endlink for details).
+ * - URL language: The language associated with URLs. When generating a URL,
+ *   this value will be used by url() as a default if no explicit preference is
+ *   provided.
+ * Modules can define additional language types through
+ * hook_language_types_info(), and alter existing language type definitions
+ * through hook_language_types_info_alter().
+ *
+ * Language types may be configurable or fixed. The language negotiation
+ * providers associated with a configurable language type can be explicitly
+ * set through the user interface. A fixed language type has predetermined
+ * (module-defined) language negotiation settings and, thus, does not appear in
+ * the configuration page. Here is a code snippet that makes the content
+ * language (which by default inherits the interface language's values)
+ * configurable:
+ * @code
+ * function mymodule_language_types_info_alter(&$language_types) {
+ *   unset($language_types[LANGUAGE_TYPE_CONTENT]['fixed']);
+ * }
+ * @endcode
+ *
+ * Every language type can have a different set of language negotiation
+ * providers assigned to it. Different language types often share the same
+ * language negotiation settings, but they can have independent settings if
+ * needed. If two language types are configured the same way, their language
+ * switcher configuration will be functionally identical and the same settings
+ * will act on both language types.
+ *
+ * Drupal defines the following built-in language negotiation providers:
+ * - URL: Determine the language from the URL (path prefix or domain).
+ * - Session: Determine the language from a request/session parameter.
+ * - User: Follow the user's language preference.
+ * - Browser: Determine the language from the browser's language settings.
+ * - Default language: Use the default site language.
+ * Language negotiation providers are simple callback functions that implement a
+ * particular logic to return a language code. For instance, the URL provider
+ * searches for a valid path prefix or domain name in the current request URL.
+ * If a language negotiation provider does not return a valid language code, the
+ * next provider associated to the language type (based on provider weight) is
+ * invoked.
+ *
+ * Modules can define additional language negotiation providers through
+ * hook_language_negotiation_info(), and alter existing providers through
+ * hook_language_negotiation_info_alter(). Here is an example snippet that lets
+ * path prefixes be ignored for administrative paths:
+ * @code
+ * function mymodule_language_negotiation_info_alter(&$negotiation_info) {
+ *   // Replace the core function with our own function.
+ *   module_load_include('language', 'inc', 'language.negotiation');
+ *   $negotiation_info[LANGUAGE_NEGOTIATION_URL]['callbacks']['negotiation'] = 'mymodule_from_url';
+ *   $negotiation_info[LANGUAGE_NEGOTIATION_URL]['file'] = drupal_get_path('module', 'mymodule') . '/mymodule.module';
+ * }
+ *
+ * function mymodule_from_url($languages) {
+ *   // Use the core URL language negotiation provider to get a valid language
+ *   // code.
+ *   module_load_include('language', 'inc', 'language.negotiation');
+ *   $langcode = language_from_url($languages);
+ *
+ *   // If we are on an administrative path, override with the default language.
+ *   if (isset($_GET['q']) && strtok($_GET['q'], '/') == 'admin') {
+ *     return language_default()->langcode;
+ *   }
+ *   return $langcode;
+ * }
+ * ?>
+ * @endcode
+ *
+ * For more information, see
+ * @link http://drupal.org/node/1497272 Language Negotiation API @endlink
+ */
+
+/**
+ * Returns all the defined language types.
  *
  * @return
  *   An array of language type names. The name will be used as the global
@@ -30,11 +121,11 @@ function language_types_info() {
 }
 
 /**
- * Return only the configurable language types.
+ * Returns only the configurable language types.
  *
  * A language type maybe configurable or fixed. A fixed language type is a type
- * whose negotiation values are unchangeable and defined while defining the
- * language type itself.
+ * whose language negotiation providers are module-defined and not altered
+ * through the user interface.
  *
  * @param $stored
  *   Optional. By default retrieves values from the 'language_types' variable to
@@ -68,7 +159,7 @@ function language_types_configurable($stored = TRUE) {
 }
 
 /**
- * Disable the given language types.
+ * Disables the given language types.
  *
  * @param $types
  *   An array of language types.
@@ -122,16 +213,17 @@ function language_types_set() {
 }
 
 /**
- * Check if a language provider is enabled.
+ * Checks whether a language negotiation provider is enabled for a language type.
  *
  * This has two possible behaviors:
  *  - If $provider_id is given return its ID if enabled, FALSE otherwise.
- *  - If no ID is passed the first enabled language provider is returned.
+ *  - If no ID is passed the first enabled language negotiation provider is
+ *    returned.
  *
  * @param $type
- *   The language negotiation type.
+ *   The language negotiation provider type.
  * @param $provider_id
- *   The language provider ID.
+ *   The language negotiation provider ID.
  *
  * @return
  *   The provider ID if it is enabled, FALSE otherwise.
@@ -155,14 +247,13 @@ function language_negotiation_get($type, $provider_id = NULL) {
 }
 
 /**
- * Check if the given language provider is enabled for any configurable language
- * type.
+ * Checks if the language negotiation provider is enabled for any language type.
  *
  * @param $provider_id
- *   The language provider ID.
+ *   The language negotiation provider ID.
  *
  * @return
- *   TRUE if there is at least one language type for which the give language
+ *   TRUE if there is at least one language type for which the given language
  *   provider is enabled, FALSE otherwise.
  */
 function language_negotiation_get_any($provider_id) {
@@ -176,7 +267,7 @@ function language_negotiation_get_any($provider_id) {
 }
 
 /**
- * Return the language switch links for the given language.
+ * Returns the language switch links for the given language.
  *
  * @param $type
  *   The language negotiation type.
@@ -223,7 +314,7 @@ function language_negotiation_get_switch_links($type, $path) {
 }
 
 /**
- * Updates language configuration to remove any language provider that is no longer defined.
+ * Removes any unused language negotation providers from the configuration.
  */
 function language_negotiation_purge() {
   // Ensure that we are getting the defined language negotiation information. An
@@ -246,12 +337,12 @@ function language_negotiation_purge() {
 }
 
 /**
- * Save a list of language providers.
+ * Saves a list of language negotiation providers.
  *
  * @param $type
  *   The language negotiation type.
  * @param $language_providers
- *   An array of language provider weights keyed by id.
+ *   An array of language negotiation provider weights keyed by provider ID.
  *   @see language_provider_weight()
  */
 function language_negotiation_set($type, $language_providers) {
@@ -277,7 +368,7 @@ function language_negotiation_set($type, $language_providers) {
       // If the provider does not express any preference about types, make it
       // available for any configurable type.
       $types = array_flip(isset($provider['types']) ? $provider['types'] : $default_types);
-      // Check if the provider is defined and has the right type.
+      // Check whether the provider is defined and has the right type.
       if (isset($types[$type])) {
         $provider_data = array();
         foreach ($provider_fields as $field) {
@@ -294,10 +385,10 @@ function language_negotiation_set($type, $language_providers) {
 }
 
 /**
- * Return all the defined language providers.
+ * Returns all the defined language negotiation providers.
  *
  * @return
- *   An array of language providers.
+ *   An array of language negotiation providers.
  */
 function language_negotiation_info() {
   $language_providers = &drupal_static(__FUNCTION__);
@@ -306,7 +397,7 @@ function language_negotiation_info() {
     // Collect all the module-defined language negotiation providers.
     $language_providers = module_invoke_all('language_negotiation_info');
 
-    // Add the default language provider.
+    // Add the default language negotiation provider.
     $language_providers[LANGUAGE_NEGOTIATION_DEFAULT] = array(
       'callbacks' => array('language' => 'language_from_default'),
       'weight' => 10,
@@ -314,7 +405,7 @@ function language_negotiation_info() {
       'description' => t('Use the default site language (@language_name).', array('@language_name' => language_default()->native)),
     );
 
-    // Let other modules alter the list of language providers.
+    // Let other modules alter the list of language negotiation providers.
     drupal_alter('language_negotiation_info', $language_providers);
   }
 
@@ -322,16 +413,17 @@ function language_negotiation_info() {
 }
 
 /**
- * Helper function used to cache the language providers results.
+ * Helper function used to cache the language negotiation providers results.
  *
  * @param $provider_id
- *   The language provider ID.
+ *   The language negotiation provider's identifier.
  * @param $provider
- *   The language provider to be invoked. If not passed it will be explicitly
- *   loaded through language_negotiation_info().
+ *   (optional) An associative array of information about the provider to be
+ *   invoked (see hook_language_negotiation_info() for details). If not passed
+ *   in, it will be loaded through language_negotiation_info().
  *
  * @return
- *   The language provider's return value.
+ *   A language object representing the language chosen by the provider.
  */
 function language_provider_invoke($provider_id, $provider = NULL) {
   $results = &drupal_static(__FUNCTION__);
@@ -352,25 +444,26 @@ function language_provider_invoke($provider_id, $provider = NULL) {
       require_once DRUPAL_ROOT . '/' . $provider['file'];
     }
 
-    // If the language provider has no cache preference or this is satisfied
-    // we can execute the callback.
+    // If the language negotiation provider has no cache preference or this is
+    // satisfied we can execute the callback.
     $cache = !isset($provider['cache']) || $user->uid || $provider['cache'] == variable_get('cache', 0);
     $callback = isset($provider['callbacks']['language']) ? $provider['callbacks']['language'] : FALSE;
     $langcode = $cache && function_exists($callback) ? $callback($languages) : FALSE;
     $results[$provider_id] = isset($languages[$langcode]) ? $languages[$langcode] : FALSE;
   }
 
-  // Since objects are resources we need to return a clone to prevent the
-  // provider cache to be unintentionally altered. The same providers might be
-  // used with different language types based on configuration.
+  // Since objects are resources, we need to return a clone to prevent the
+  // language negotiation provider cache from being unintentionally altered. The
+  // same providers might be used with different language types based on
+  // configuration.
   return !empty($results[$provider_id]) ? clone($results[$provider_id]) : $results[$provider_id];
 }
 
 /**
- * Return the passed language provider weight or a default value.
+ * Returns the passed language negotiation provider weight or a default value.
  *
  * @param $provider
- *   A language provider data structure.
+ *   A language negotiation provider data structure.
  *
  * @return
  *   A numeric weight.
@@ -381,16 +474,16 @@ function language_provider_weight($provider) {
 }
 
 /**
- * Choose a language for the given type based on language negotiation settings.
+ * Chooses a language based on language negotiation provider settings.
  *
  * @param $type
- *   The language type.
+ *   The language type key to find the language for.
  *
  * @return
  *   The negotiated language object.
  */
 function language_initialize($type) {
-  // Execute the language providers in the order they were set up and return the
+  // Execute the language negotiation providers in the order they were set up and return the
   // first valid language found.
   $negotiation = variable_get("language_negotiation_$type", array());
 
@@ -409,7 +502,7 @@ function language_initialize($type) {
 }
 
 /**
- * Default language provider.
+ * Returns the default language negotiation provider.
  *
  * @return
  *   The default language code.
@@ -421,8 +514,8 @@ function language_from_default() {
 /**
  * Splits the given path into prefix and actual path.
  *
- * Parse the given path and return the language object identified by the
- * prefix and the actual path.
+ * Parse the given path and return the language object identified by the prefix
+ * and the actual path.
  *
  * @param $path
  *   The path to split.
@@ -482,3 +575,7 @@ function language_fallback_get_candidates($type = LANGUAGE_TYPE_CONTENT) {
 
   return $fallback_candidates;
 }
+
+/**
+ * @} End of "language_negotiation"
+ */
diff --git a/modules/system/language.api.php b/modules/system/language.api.php
index 671479309be0a8e90643e2b337f5cbacf03132e6..d868b6fef30a68b293e4192765701213a1bc0684 100644
--- a/modules/system/language.api.php
+++ b/modules/system/language.api.php
@@ -62,16 +62,22 @@ function hook_language_switch_links_alter(array &$links, $type, $path) {
 }
 
 /**
- * Allow modules to define their own language types.
+ * Define language types.
  *
  * @return
- *   An array of language type definitions. Each language type has an identifier
- *   key. The language type definition is an associative array that may contain
- *   the following key-value pairs:
- *   - "name": The human-readable language type identifier.
- *   - "description": A description of the language type.
- *   - "fixed": An array of language provider identifiers. Defining this key
- *     makes the language type non-configurable.
+ *   An associative array of language type definitions. The keys are the
+ *   identifiers, which are also used as names for global variables representing
+ *   the types in the bootstrap phase. The values are associative arrays that
+ *   may contain the following elements:
+ *   - name: The human-readable language type identifier.
+ *   - description: A description of the language type.
+ *   - fixed: A fixed array of language negotiation provider identifiers to use
+ *     to initialize this language. Defining this key makes the language type
+ *     non-configurable, so it will always use the specified providers in the
+ *     given priority order. Omit to make the language type configurable.
+ *
+ * @see hook_language_types_info_alter()
+ * @ingroup language_negotiation
  */
 function hook_language_types_info() {
   return array(
@@ -90,6 +96,9 @@ function hook_language_types_info() {
  *
  * @param $language_types
  *   Array of language type definitions.
+ *
+ * @see hook_language_types_info()
+ * @ingroup language_negotiation
  */
 function hook_language_types_info_alter(array &$language_types) {
   if (isset($language_types['custom_language_type'])) {
@@ -98,31 +107,35 @@ function hook_language_types_info_alter(array &$language_types) {
 }
 
 /**
- * Allow modules to define their own language providers.
+ * Define language negotiation providers.
  *
  * @return
- *   An array of language provider definitions. Each language provider has an
- *   identifier key. The language provider definition is an associative array
- *   that may contain the following key-value pairs:
- *   - "types": An array of allowed language types. If a language provider does
- *     not specify which language types it should be used with, it will be
- *     available for all the configurable language types.
- *   - "callbacks": An array of functions that will be called to perform various
- *     tasks. Possible key-value pairs are:
- *     - "language": Required. The callback that will determine the language
- *       value.
- *     - "switcher": The callback that will determine the language switch links
- *       associated to the current language provider.
- *     - "url_rewrite": The callback that will provide URL rewriting.
- *   - "file": A file that will be included before the callback is invoked; this
- *     allows callback functions to be in separate files.
- *   - "weight": The default weight the language provider has.
- *   - "name": A human-readable identifier.
- *   - "description": A description of the language provider.
- *   - "config": An internal path pointing to the language provider
- *     configuration page.
- *   - "cache": The value Drupal's page cache should be set to for the current
- *     language provider to be invoked.
+ *   An associative array of language negotiation provider definitions. The keys
+ *   are provider identifiers, and the values are associative arrays definining
+ *   each provider, with the following elements:
+ *   - types: An array of allowed language types. If a language negotiation
+ *     provider does not specify which language types it should be used with, it
+ *     will be available for all the configurable language types.
+ *   - callbacks: An associative array of functions that will be called to
+ *     perform various tasks. Possible elements are:
+ *     - negotiation: (required) Name of the callback function that determines
+ *       the language value.
+ *     - language_switch: (optional) Name of the callback function that
+ *       determines links for a language switcher block associated with this
+ *       provider. See language_switcher_url() for an example.
+ *     - url_rewrite: (optional) Name of the callback function that provides URL
+ *       rewriting, if needed by this provider.
+ *   - file: The file where callback functions are defined (this file will be
+ *     included before the callbacks are invoked).
+ *   - weight: The default weight of the provider.
+ *   - name: The translated human-readable name for the provider.
+ *   - description: A translated longer description of the provider.
+ *   - config: An internal path pointing to the provider's configuration page.
+ *   - cache: The value Drupal's page cache should be set to for the current
+ *     provider to be invoked.
+ *
+ * @see hook_language_negotiation_info_alter()
+ * @ingroup language_negotiation
  */
 function hook_language_negotiation_info() {
   return array(
@@ -135,18 +148,21 @@ function hook_language_negotiation_info() {
       'file' => drupal_get_path('module', 'custom') . '/custom.module',
       'weight' => -4,
       'types' => array('custom_language_type'),
-      'name' => t('Custom language provider'),
-      'description' => t('This is a custom language provider.'),
+      'name' => t('Custom language negotiation provider'),
+      'description' => t('This is a custom language negotiation provider.'),
       'cache' => 0,
     ),
   );
 }
 
 /**
- * Perform alterations on language providers.
+ * Perform alterations on language negoiation providers.
  *
  * @param $language_providers
- *   Array of language provider definitions.
+ *   Array of language negotiation provider definitions.
+ *
+ * @see hook_language_negotiation_info()
+ * @ingroup language_negotiation
  */
 function hook_language_negotiation_info_alter(array &$language_providers) {
   if (isset($language_providers['custom_language_provider'])) {