diff --git a/nova/api/openstack/wsgi.py b/nova/api/openstack/wsgi.py
index 8f7cafbc326c..e95cb612d395 100644
--- a/nova/api/openstack/wsgi.py
+++ b/nova/api/openstack/wsgi.py
@@ -96,8 +96,10 @@ class Request(wsgi.Request):
                     content_type = possible_type
 
             if not content_type:
-                content_type = self.accept.best_match(
+                best_matches = self.accept.acceptable_offers(
                     get_supported_content_types())
+                if best_matches:
+                    content_type = best_matches[0][0]
 
             self.environ['nova.best_content_type'] = (content_type or
                                                       'application/json')
@@ -134,8 +136,19 @@ class Request(wsgi.Request):
         """
         if not self.accept_language:
             return None
-        return self.accept_language.best_match(
-                i18n.get_available_languages())
+
+        # NOTE(takashin): To decide the default behavior, 'default' is
+        # preferred over 'default_tag' because that is return as it is when
+        # no match. This is also little tricky that 'default' value cannot be
+        # None. At least one of default_tag or default must be supplied as
+        # an argument to the method, to define the defaulting behavior.
+        # So passing a sentinal value to return None from this function.
+        best_match = self.accept_language.lookup(
+            i18n.get_available_languages(), default='fake_LANG')
+
+        if best_match == 'fake_LANG':
+            best_match = None
+        return best_match
 
     def set_api_version_request(self):
         """Set API version request based on the request header information."""
diff --git a/nova/tests/unit/api/openstack/test_wsgi.py b/nova/tests/unit/api/openstack/test_wsgi.py
index bf6dded7efc3..0078ea5fe7b8 100644
--- a/nova/tests/unit/api/openstack/test_wsgi.py
+++ b/nova/tests/unit/api/openstack/test_wsgi.py
@@ -50,7 +50,7 @@ class RequestTest(MicroversionedTest):
         super(RequestTest, self).setUp()
         self.stub_out('nova.i18n.get_available_languages',
                       lambda *args, **kwargs:
-                      ['en_GB', 'en_AU', 'de', 'zh_CN', 'en_US'])
+                      ['en-GB', 'en-AU', 'de', 'zh-CN', 'en-US', 'ja-JP'])
 
     def test_content_type_missing(self):
         request = wsgi.Request.blank('/tests/123', method='POST')
@@ -76,31 +76,49 @@ class RequestTest(MicroversionedTest):
         result = request.best_match_content_type()
         self.assertEqual(result, "application/json")
 
+    def test_content_type_accept_with_quality_values(self):
+        request = wsgi.Request.blank('/tests/123')
+        request.headers["Accept"] = (
+            "application/json;q=0.4,"
+            "application/vnd.openstack.compute+json;q=0.6")
+        result = request.best_match_content_type()
+        self.assertEqual("application/vnd.openstack.compute+json", result)
+
     def test_from_request(self):
         request = wsgi.Request.blank('/')
         accepted = 'bogus;q=1, en-gb;q=0.7,en-us,en;q=0.5,*;q=0.7'
         request.headers = {'Accept-Language': accepted}
-        self.assertEqual(request.best_match_language(), 'en_US')
+        self.assertEqual(request.best_match_language(), 'en-US')
 
     def test_asterisk(self):
-        # asterisk should match first available if there
-        # are not any other available matches
+        # In the section 3.4 of RFC4647, it says as follows:
+        # If the language range "*"(asterisk) is the only one
+        # in the language priority list or if no other language range
+        # follows, the default value is computed and returned.
+        #
+        # In this case, the default value 'None' is returned.
         request = wsgi.Request.blank('/')
-        accepted = '*,es;q=0.5'
+        accepted = '*;q=0.5'
         request.headers = {'Accept-Language': accepted}
-        self.assertEqual(request.best_match_language(), 'en_GB')
+        self.assertIsNone(request.best_match_language())
 
-    def test_prefix(self):
+    def test_asterisk_followed_by_other_language(self):
         request = wsgi.Request.blank('/')
-        accepted = 'zh'
+        accepted = '*,ja-jp;q=0.5'
         request.headers = {'Accept-Language': accepted}
-        self.assertEqual(request.best_match_language(), 'zh_CN')
+        self.assertEqual('ja-JP', request.best_match_language())
+
+    def test_truncate(self):
+        request = wsgi.Request.blank('/')
+        accepted = 'de-CH'
+        request.headers = {'Accept-Language': accepted}
+        self.assertEqual('de', request.best_match_language())
 
     def test_secondary(self):
         request = wsgi.Request.blank('/')
         accepted = 'nn,en-gb;q=0.5'
         request.headers = {'Accept-Language': accepted}
-        self.assertEqual(request.best_match_language(), 'en_GB')
+        self.assertEqual('en-GB', request.best_match_language())
 
     def test_none_found(self):
         request = wsgi.Request.blank('/')