diff --git a/includes/language.inc b/includes/language.inc
index eb13eac..8a2c791 100644
--- a/includes/language.inc
+++ b/includes/language.inc
@@ -84,6 +84,42 @@ function language_types_disable($types) {
 }
 
 /**
+ * Update the language type configuration.
+ *
+ * @param $defined_providers
+ *   (Optional) A list of defined language providers. Defaults to the
+ *   system-defined ones.
+ */
+function language_types_set($defined_providers = NULL) {
+  if (empty($defined_providers)) {
+    drupal_static_reset('language_negotiation_info');
+    $defined_providers = language_negotiation_info();
+  }
+
+  drupal_static_reset('language_types_info');
+  foreach (language_types_info() as $type => $info) {
+    if (isset($info['fixed'])) {
+      $language_types[$type] = FALSE;
+      $negotiation = array();
+      foreach ($info['fixed'] as $weight => $id) {
+        if (isset($defined_providers[$id])) {
+          $negotiation[$id] = $defined_providers[$id];
+          $negotiation[$id]['weight'] = $weight;
+        }
+      }
+      language_negotiation_set($type, $negotiation);
+    }
+    else {
+      $language_types[$type] = TRUE;
+    }
+  }
+
+  // Save language types.
+  variable_set('language_types', $language_types);
+  drupal_static_reset('language_types_configurable');
+}
+
+/**
  * Check if a language provider is enabled.
  *
  * This has two possible behaviors:
@@ -173,6 +209,30 @@ function language_negotiation_get_switch_links($type, $path) {
   return $links;
 }
 
+/**
+ * Ensure that the enabled language providers are actually defined.
+ *
+ * @param $defined_providers
+ *   (Optional) A list of defined language providers. Defaults to the
+ *   system-defined ones.
+ */
+function language_negotiation_purge($defined_providers = NULL) {
+  if (empty($defined_providers)) {
+    drupal_static_reset('language_negotiation_info');
+    $defined_providers = language_negotiation_info();
+  }
+
+  drupal_static_reset('language_types_info');
+  foreach (language_types_info() as $type => $type_info) {
+    $negotiation = array();
+    foreach (variable_get("language_negotiation_$type", array()) as $id => $provider) {
+      if (isset($defined_providers[$id])) {
+        $negotiation[] = $id;
+      }
+    }
+    language_negotiation_set($type, array_flip($negotiation));
+  }
+}
 
 /**
  * Save a list of language providers.
@@ -180,7 +240,8 @@ function language_negotiation_get_switch_links($type, $path) {
  * @param $type
  *   The language negotiation type.
  * @param $language_providers
- *   An array of language provider ids.
+ *   An array of language provider weights keyed by id.
+ *   @see language_provider_weight()
  */
 function language_negotiation_set($type, $language_providers) {
   // Save only the necessary fields.
@@ -189,7 +250,7 @@ function language_negotiation_set($type, $language_providers) {
   $negotiation = array();
   $providers_weight = array();
   $defined_providers = language_negotiation_info();
-  $default_types = language_types_configurable();
+  $default_types = language_types_configurable(FALSE);
 
   // Initialize the providers weight list.
   foreach ($language_providers as $id => $provider) {
diff --git a/modules/locale/locale.admin.inc b/modules/locale/locale.admin.inc
index de16133..3331c69 100644
--- a/modules/locale/locale.admin.inc
+++ b/modules/locale/locale.admin.inc
@@ -541,6 +541,11 @@ function _locale_languages_configure_form_language_table(&$form, $type) {
   asort($providers_weight);
 
   foreach ($providers_weight as $id => $weight) {
+    // Exclude no more available providers.
+    if (!isset($language_providers[$id])) {
+      continue;
+    }
+
     $enabled = isset($enabled_providers[$id]);
     $provider = $language_providers[$id];
 
@@ -658,7 +663,6 @@ function theme_locale_languages_configure_form($variables) {
  * Submit handler for language negotiation settings.
  */
 function locale_languages_configure_form_submit($form, &$form_state) {
-  $language_types = array();
   $configurable_types = $form['#language_types'];
 
   foreach ($configurable_types as $type) {
@@ -666,7 +670,6 @@ function locale_languages_configure_form_submit($form, &$form_state) {
     $enabled_providers = $form_state['values'][$type]['enabled'];
     $enabled_providers[LANGUAGE_NEGOTIATION_DEFAULT] = TRUE;
     $providers_weight = $form_state['values'][$type]['weight'];
-    $language_types[$type] = TRUE;
 
     foreach ($providers_weight as $id => $weight) {
       if ($enabled_providers[$id]) {
@@ -680,27 +683,11 @@ function locale_languages_configure_form_submit($form, &$form_state) {
     variable_set("locale_language_providers_weight_$type", $providers_weight);
   }
 
-  // Save non-configurable language types negotiation.
-  $language_types_info = language_types_info();
-  $defined_providers = $form['#language_providers'];
-  foreach ($language_types_info as $type => $info) {
-    if (isset($info['fixed'])) {
-      $language_types[$type] = FALSE;
-      $negotiation = array();
-      foreach ($info['fixed'] as $weight => $id) {
-        if (isset($defined_providers[$id])) {
-          $negotiation[$id] = $defined_providers[$id];
-          $negotiation[$id]['weight'] = $weight;
-        }
-      }
-      language_negotiation_set($type, $negotiation);
-    }
-  }
-
-  // Save language types.
-  variable_set('language_types', $language_types);
+  // Save non-configurable language types and the related language negotiation
+  // configuration.
+  language_types_set($form['#language_providers']);
 
-  $form_state['redirect'] = 'admin/config/regional/language';
+  $form_state['redirect'] = 'admin/config/regional/language/configure';
   drupal_set_message(t('Language negotiation configuration saved.'));
 }
 
diff --git a/modules/locale/locale.module b/modules/locale/locale.module
index c1cdd43..13cb233 100644
--- a/modules/locale/locale.module
+++ b/modules/locale/locale.module
@@ -513,6 +513,8 @@ function locale_language_types_info() {
       'description' => t('Order of language detection methods for user interface text. If a translation of user interface text is available in the detected language, it will be displayed.'),
     ),
     LANGUAGE_TYPE_CONTENT => array(
+      'name' => t('Content'),
+      'description' => t('Order of language detection methods for content. If a version of content is available in the detected language, it will be displayed.'),
       'fixed' => array(LOCALE_LANGUAGE_NEGOTIATION_INTERFACE),
     ),
     LANGUAGE_TYPE_URL => array(
@@ -593,6 +595,22 @@ function locale_language_negotiation_info() {
   return $providers;
 }
 
+/**
+ * Implementation of hook_modules_enabled().
+ */
+function locale_modules_enabled($modules) {
+  include_once DRUPAL_ROOT . '/includes/language.inc';
+  language_types_set();
+  language_negotiation_purge();
+}
+
+/**
+ * Implementation of hook_modules_disabled().
+ */
+function locale_modules_disabled($modules) {
+  locale_modules_enabled($modules);
+}
+
 // ---------------------------------------------------------------------------------
 // Locale core functionality
 
@@ -928,7 +946,7 @@ function locale_block_info() {
   include_once DRUPAL_ROOT . '/includes/language.inc';
   $block = array();
   $info = language_types_info();
-  foreach (language_types_configurable() as $type) {
+  foreach (language_types_configurable(FALSE) as $type) {
     $block[$type] = array(
       'info' => t('Language switcher (@type)', array('@type' => $info[$type]['name'])),
       // Not worth caching.
diff --git a/modules/locale/locale.test b/modules/locale/locale.test
index 5e9e833..2fef001 100644
--- a/modules/locale/locale.test
+++ b/modules/locale/locale.test
@@ -17,6 +17,7 @@
  *  - a functional test for multilingual support by content type and on nodes.
  *  - a functional test for multilingual fields.
  *  - a functional test for comment language.
+ *  - a functional test fot language types/negotiation info.
  */
 
 
@@ -2248,10 +2249,13 @@ class LocaleCommentLanguageFunctionalTest extends DrupalWebTestCase {
     variable_set('locale_test_content_language_type', TRUE);
 
     // Set interface language detection to user and content language detection
-    // to URL.
+    // to URL. Disable inheritance from interface language to ensure content
+    // language will fall back to the default language if no URL language can be
+    // detected.
     $edit = array(
       'language[enabled][locale-user]' => TRUE,
-      'language_content[enabled][locale-url]' => TRUE
+      'language_content[enabled][locale-url]' => TRUE,
+      'language_content[enabled][locale-interface]' => FALSE,
     );
     $this->drupalPost('admin/config/regional/language/configure', $edit, t('Save settings'));
 
@@ -2373,3 +2377,158 @@ class LocaleDateFormatsFunctionalTest extends DrupalWebTestCase {
     $this->assertText($french_date, t('French date format appears'));
   }
 }
+
+/**
+ * Functional test for language types/negotiation info.
+ */
+class LocaleLanguageNegotiationInfoFunctionalTest extends DrupalWebTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Language negotiation info',
+      'description' => 'Tests alterations to language types/negotiation info.',
+      'group' => 'Locale',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('locale');
+    require_once DRUPAL_ROOT .'/includes/language.inc';
+    $admin_user = $this->drupalCreateUser(array('administer languages', 'access administration pages', 'view the administration theme'));
+    $this->drupalLogin($admin_user);
+    $this->drupalPost('admin/config/regional/language/add', array('langcode' => 'it'), t('Add language'));
+  }
+
+  /**
+   * Tests alterations to language types/negotiation info.
+   */
+  function testInfoAlterations() {
+    // Enable language type/negotiation info alterations.
+    variable_set('locale_test_language_types', TRUE);
+    variable_set('locale_test_language_negotiation_info', TRUE);
+    $this->languageNegotiationUpdate();
+
+    // Check that fixed language types are properly configured without the need
+    // of saving the language negotiation settings.
+    $this->checkFixedLanguageTypes();
+
+    // Make the content language type configurable by updating the language
+    // negotiation settings with the proper flag enabled.
+    variable_set('locale_test_content_language_type', TRUE);
+    $this->languageNegotiationUpdate();
+    $type = LANGUAGE_TYPE_CONTENT;
+    $language_types = variable_get('language_types', drupal_language_types());
+    $this->assertTrue($language_types[$type], t('Content language type is configurable.'));
+
+    // Enable some core and custom language providers. The test language type is
+    // supposed to be configurable.
+    $test_type = 'test_language_type';
+    $provider = LOCALE_LANGUAGE_NEGOTIATION_INTERFACE;
+    $test_provider = 'test_language_provider';
+    $form_field = $type . '[enabled]['. $provider .']';
+    $edit = array(
+      $form_field => TRUE,
+      $type . '[enabled][' . $test_provider . ']' => TRUE,
+      $test_type . '[enabled][' . $test_provider . ']' => TRUE,
+    );
+    $this->drupalPost('admin/config/regional/language/configure', $edit, t('Save settings'));
+
+    // Remove the interface language provider by updating the language
+    // negotiation settings with the proper flag enabled.
+    variable_set('locale_test_language_negotiation_info_alter', TRUE);
+    $this->languageNegotiationUpdate();
+    $negotiation = variable_get("language_negotiation_$type", array());
+    $this->assertFalse(isset($negotiation[$provider]), t('Interface language provider removed from the stored settings.'));
+    $this->assertNoFieldByXPath("//input[@name=\"$form_field\"]", NULL, t('Interface language provider unavailable.'));
+
+    // Check that type-specific language providers can be assigned only to the
+    // corresponding language types.
+    foreach (language_types_configurable() as $type) {
+      $form_field = $type . '[enabled][test_language_provider_ts]';
+      if ($type == $test_type) {
+        $this->assertFieldByXPath("//input[@name=\"$form_field\"]", NULL, t('Type-specific test language provider available for %type.', array('%type' => $type)));
+      }
+      else {
+        $this->assertNoFieldByXPath("//input[@name=\"$form_field\"]", NULL, t('Type-specific test language provider unavailable for %type.', array('%type' => $type)));
+      }
+    }
+
+    // Check language negotiation results.
+    $this->drupalGet('');
+    $last = variable_get('locale_test_language_negotiation_last', array());
+    foreach (language_types() as $type) {
+      $langcode = $last[$type];
+      $value = $type == LANGUAGE_TYPE_CONTENT || strpos($type, 'test') !== FALSE ? 'it' : 'en';
+      $this->assertEqual($langcode, $value, t('The negotiated language for %type is %language', array('%type' => $type, '%language' => $langcode)));
+    }
+
+    // Disable locale_test and check that everything is set back to the original
+    // status.
+    $this->languageNegotiationUpdate('disable');
+
+    // Check that only the core language types are available.
+    foreach (language_types() as $type) {
+      $this->assertTrue(strpos($type, 'test') === FALSE, t('The %type language is still available', array('%type' => $type)));
+    }
+
+    // Check that fixed language types are properly configured, even those
+    // previously set to configurable.
+    $this->checkFixedLanguageTypes();
+
+    // Check that unavailable language providers are not present in the
+    // negotiation settings.
+    $negotiation = variable_get("language_negotiation_$type", array());
+    $this->assertFalse(isset($negotiation[$test_provider]), t('The disabled test language provider is not part of the content language negotiation settings.'));
+
+    // Check that configuration page presents the correct options and settings.
+    $this->assertNoRaw(t('Test language detection'), t('No test language type configuration available.'));
+    $this->assertNoRaw(t('This is a test language provider'), t('No test language provider available.'));
+  }
+
+  /**
+   * Update language types/negotiation information.
+   *
+   * Manually invoke locale_modules_enabled()/locale_modules_disabled() since
+   * they would not be invoked after enabling/disabling locale_test the first
+   * time.
+   */
+  private function languageNegotiationUpdate($op = 'enable') {
+    static $last_op = NULL;
+    $modules = array('locale_test');
+
+    // Enable/disable locale_test only we did not alreay before.
+    if ($last_op != $op) {
+      $function = "module_{$op}";
+      $function($modules);
+      // Reset hook implementation cache.
+      module_implements(NULL, FALSE, TRUE);
+    }
+
+    drupal_static_reset('language_types_info');
+    drupal_static_reset('language_negotiation_info');
+    $function = "locale_modules_{$op}d";
+    if (function_exists($function)) {
+      $function($modules);
+    }
+
+    $this->drupalGet('admin/config/regional/language/configure');
+  }
+
+  /**
+   * Check that language negotiation for fixed types matches the stored one.
+   */
+  private function checkFixedLanguageTypes() {
+    drupal_static_reset('language_types_info');
+    foreach (language_types_info() as $type => $info) {
+      if (isset($info['fixed'])) {
+        $negotiation = variable_get("language_negotiation_$type", array());
+        $equal = count($info['fixed']) == count($negotiation);
+        while ($equal && list($id) = each($negotiation)) {
+          list(, $info_id) = each($info['fixed']);
+          $equal = $info_id == $id;
+        }
+        $this->assertTrue($equal, t('language negotiation for %type is properly set up', array('%type' => $type)));
+      }
+    }
+  }
+}
diff --git a/modules/locale/tests/locale_test.module b/modules/locale/tests/locale_test.module
index f256b5c..14a2588 100644
--- a/modules/locale/tests/locale_test.module
+++ b/modules/locale/tests/locale_test.module
@@ -28,13 +28,88 @@ function locale_test_boot() {
 }
 
 /**
+ * Implements hook_init().
+ */
+function locale_test_init() {
+  locale_test_store_language_negotiation();
+}
+
+/**
+ * Implements hook_language_types_info().
+ */
+function locale_test_language_types_info() {
+  if (variable_get('locale_test_language_types', FALSE)) {
+    return array(
+      'test_language_type' => array(
+        'name' => t('Test'),
+        'description' => t('A test language type.'),
+      ),
+      'fixed_test_language_type' => array(
+        'fixed' => array('test_language_provider'),
+      ),
+    );
+  }
+}
+
+/**
  * Implements hook_language_types_info_alter().
  */
 function locale_test_language_types_info_alter(array &$language_types) {
   if (variable_get('locale_test_content_language_type', FALSE)) {
-    $language_types[LANGUAGE_TYPE_CONTENT] = array(
-      'name' => t('Content'),
-      'description' => t('Order of language detection methods for content. If a version of content is available in the detected language, it will be displayed.'),
+    unset($language_types[LANGUAGE_TYPE_CONTENT]['fixed']);
+  }
+}
+
+/**
+ * Implements hook_language_negotiation_info().
+ */
+function locale_test_language_negotiation_info() {
+  if (variable_get('locale_test_language_negotiation_info', FALSE)) {
+    $info = array(
+      'callbacks' => array(
+        'language' => 'locale_test_language_provider',
+      ),
+      'file' => drupal_get_path('module', 'locale_test') .'/locale_test.module',
+      'weight' => -10,
+      'description' => t('This is a test language provider.'),
     );
+
+    return array(
+      'test_language_provider' => array(
+        'name' => t('Test'),
+        'types' => array(LANGUAGE_TYPE_CONTENT, 'test_language_type', 'fixed_test_language_type'),
+      ) + $info,
+      'test_language_provider_ts' => array(
+        'name' => t('Type-specific test'),
+        'types' => array('test_language_type'),
+      ) + $info,
+    );
+  }
+}
+
+/**
+ * Implements hook_language_negotiation_info_alter().
+ */
+function locale_test_language_negotiation_info_alter(array &$language_providers) {
+  if (variable_get('locale_test_language_negotiation_info_alter', FALSE)) {
+    unset($language_providers[LOCALE_LANGUAGE_NEGOTIATION_INTERFACE]);
   }
 }
+
+/**
+ * Store the last negotiated languages.
+ */
+function locale_test_store_language_negotiation() {
+  $last = array();
+  foreach (language_types() as $type) {
+    $last[$type] = $GLOBALS[$type]->language;
+  }
+  variable_set('locale_test_language_negotiation_last', $last);
+}
+
+/**
+ * Test language provider.
+ */
+function locale_test_language_provider($languages) {
+  return 'it';
+}