diff --git a/modules/locale/locale.test b/modules/locale/locale.test
index 03c4bcf337729a6c4fdd5cf231ffc37721b69f2c..47f52cd4935ddccf85ca3d14b1fa16e078bb3e4d 100644
--- a/modules/locale/locale.test
+++ b/modules/locale/locale.test
@@ -1408,3 +1408,195 @@ class LocaleContentFunctionalTest extends DrupalWebTestCase {
   }
 }
 
+
+
+/**
+ * Test UI language negotiation
+ * 1. LANGUAGE_NEGOTIATION_PATH_DEFAULT
+ *    UI Language base on URL prefix, browser language preference has no
+ *    influence:
+ *      admin/config
+ *        UI in site default language
+ *      zh-hans/admin/config
+ *        UI in Chinese
+ *      blah-blah/admin/config
+ *        404
+ * 2. LANGUAGE_NEGOTIATION_PATH
+ *        admin/config
+ *          UI in user's browser language preference if the site has that
+ *          language enabled, if not, the default language
+ *        zh-hans/admin/config
+ *          UI in Chinese
+ *        blah-blah/admin/config
+ *          404
+ * 3. LANGUAGE_NEGOTIATION_DOMAIN
+ *        http://example.com/admin/config
+ *          UI language in site default
+ *        http://example.cn/admin/config
+ *          UI language in Chinese
+ */
+class UILanguageNegotiationTest extends DrupalWebTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'UI language negotiation',
+      'description' => 'Test UI language switching by url path prefix and domain.',
+      'group' => 'Locale',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('locale', 'locale_test');
+  }
+
+  /**
+   * Tests for language switching by URL path.
+   */
+  function testUILanguageNegotiation() {
+    $admin_user = $this->drupalCreateUser(array('administer languages', 'translate interface', 'access administration pages'));
+    $this->drupalLogin($admin_user);
+
+    // A few languages to switch to.
+    // This one is unknown, should get the default lang version.
+    $language_unknown = 'blah-blah';
+    // For testing browser lang preference.
+    $language_browser_fallback = 'vi';
+    // For testing path prefix.
+    $language = 'zh-hans';
+    // For setting browser language preference to 'vi'.
+    $http_header_browser_fallback = array("Accept-Language: $language_browser_fallback;q=1");
+    // For setting browser language preference to some unknown.
+    $http_header_blah = array("Accept-Language: blah;q=1");
+
+    // This domain should switch the UI to Chinese.
+    $language_domain = 'example.cn';
+
+    // Setup the site languages by installing two languages.
+    require_once('includes/locale.inc');
+    locale_add_language($language_browser_fallback);
+    locale_add_language($language);
+
+    // We will look for this string in the admin/config screen to see if the
+    // corresponding translated string is shown.
+    $default_string = 'Configure languages for content and the user interface';
+
+    // Set the default language in order for the translated string to be registered
+    // into database when seen by t(). Without doing this, our target string
+    // is for some reason not found when doing translate search. This might
+    // be some bug.
+    $languages = language_list('enabled');
+    variable_set('language_default', $languages[1]['vi']);
+    // First visit this page to make sure our target string is searchable.
+    $this->drupalGet('admin/config');
+    // Now the t()'ed string is in db so switch the language back to default.
+    variable_del('language_default');
+
+    // Translate the string.
+    $language_browser_fallback_string = "In $language_browser_fallback In $language_browser_fallback In $language_browser_fallback";
+    $language_string = "In $language In $language In $language";
+    // Do a translate search of our target string.
+    $edit = array( 'string' => $default_string);
+    $this->drupalPost('admin/config/international/translate/translate', $edit, t('Filter'));
+    // Should find the string and now click edit to post translated string.
+    $this->clickLink('edit');
+    $edit = array(
+      "translations[$language_browser_fallback]" => $language_browser_fallback_string,
+      "translations[$language]" => $language_string,
+    );
+    $this->drupalPost(NULL, $edit, t('Save translations'));
+
+    $tests = array(
+      // Default, browser preference should have no influence.
+      array(
+        'language_negotiation' => LANGUAGE_NEGOTIATION_PATH_DEFAULT,
+        'path' => 'admin/config',
+        'expect' => $default_string,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'LANGUAGE_NEGOTIATION_PATH_DEFAULT: no language prefix, UI language is default and not the browser language preference setting is used.',
+      ),
+      // Language prefix.
+      array(
+        'language_negotiation' => LANGUAGE_NEGOTIATION_PATH_DEFAULT,
+        'path' => "$language/admin/config",
+        'expect' => $language_string,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'LANGUAGE_NEGOTIATION_PATH_DEFAULT: with language prefix, UI language is switched based on path prefix',
+      ),
+      // Default, go by browser preference.
+      array(
+        'language_negotiation' => LANGUAGE_NEGOTIATION_PATH,
+        'path' => 'admin/config',
+        'expect' => $language_browser_fallback_string,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'LANGUAGE_NEGOTIATION_PATH: no language prefix, UI language is determined by browser language preference',
+      ),
+      // Prefix, switch to the language.
+      array(
+        'language_negotiation' => LANGUAGE_NEGOTIATION_PATH,
+        'path' => "$language/admin/config",
+        'expect' => $language_string,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'LANGUAGE_NEGOTIATION_PATH: with langage prefix, UI language is based on path prefix',
+      ),
+      // Default, browser language preference is not one of site's lang.
+      array(
+        'language_negotiation' => LANGUAGE_NEGOTIATION_PATH,
+        'path' => 'admin/config',
+        'expect' => $default_string,
+        'http_header' => $http_header_blah,
+        'message' => 'LANGUAGE_NEGOTIATION_PATH: no language prefix and browser language preference set to unknown language should use default language',
+      ),
+    );
+
+    foreach ($tests as $test) {
+      $this->runTest($test);
+    }
+
+    // Unknown language prefix should return 404.
+    foreach(array(LANGUAGE_NEGOTIATION_PATH_DEFAULT, LANGUAGE_NEGOTIATION_PATH) as $negotiation) {
+      variable_set('language_negotiation', $negotiation);
+      $this->drupalGet("$language_unknown/admin/config", array(), $http_header_browser_fallback);
+      $this->assertResponse(404, "Unknown language path prefix should return 404, code = $negotiation");
+    }
+
+    // Setup for domain negotiation, first configure the language to have domain
+    // URL.
+    $edit = array('prefix' => '', 'domain' => "http://$language_domain");
+    $this->drupalPost("admin/config/international/language/edit/$language", $edit, t('Save language'));
+    // Set the site to use domain language negotiation.
+    variable_set('language_negotiation', LANGUAGE_NEGOTIATION_DOMAIN);
+
+    $tests = array(
+      // Default domain, browser preference should have no influence.
+      array(
+        'path' => 'admin/config',
+        'expect' => $default_string,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'LANGUAGE_NEGOTIATION_DOMAIN: default domain should get default language',
+      ),
+      // Language domain specific URL, we set the $_SERVER['HTTP_HOST'] in
+      // locale_test.module hook_boot() to simulate this.
+      array(
+        'locale_test_domain' => $language_domain,
+        'path' => 'admin/config',
+        'expect' => $language_string,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'LANGUAGE_NEGOTIATION_DOMAIN: domain example.cn should switch to Chinese',
+      ),
+    );
+
+    foreach ($tests as $test) {
+      $this->runTest($test);
+    }
+  }
+
+  private function runTest($test) {
+    if (!empty($test['language_negotiation'])) {
+      variable_set('language_negotiation', $test['language_negotiation']);
+    }
+    if (!empty($test['locale_test_domain'])) {
+      variable_set('locale_test_domain', $test['locale_test_domain']);
+    }
+    $this->drupalGet($test['path'], array(), $test['http_header']);
+    $this->assertText($test['expect'], $test['message']);
+  }
+}
diff --git a/modules/locale/tests/locale_test.module b/modules/locale/tests/locale_test.module
index 87423f04b822ee577fd7562ee98527bce70ec37a..ca0524ab95aaf959714a6404f8164ee933890435 100644
--- a/modules/locale/tests/locale_test.module
+++ b/modules/locale/tests/locale_test.module
@@ -15,3 +15,13 @@ function locale_test_locale($op = 'groups') {
       return array('custom' => t('Custom'));
   }
 }
+
+/**
+ * For testing domain language negotiation, we fake it by setting
+ * the HTTP_HOST here
+ */
+function locale_test_boot() {
+  if (variable_get('locale_test_domain')) {
+    $_SERVER['HTTP_HOST'] = variable_get('locale_test_domain');
+  }
+}