Honor browser defined langs

Add tests for PR13441

Apply DRY principle, rename test function name

openedx/core/djangoapps/lang_pref/middleware.py

@@ -39,9 +39,15 @@ class LanguagePreferenceMiddleware(object):
             preferred_language = request.META.get('HTTP_ACCEPT_LANGUAGE', '')
             lang_headers = [seq[0] for seq in parse_accept_lang_header(preferred_language)]
 
+            prefixes = [prefix.split("-")[0] for prefix in system_released_languages]
             # Setting the session language to the browser language, if it is supported.
             for browser_lang in lang_headers:
                 if browser_lang in system_released_languages:
-                    if request.session.get(LANGUAGE_SESSION_KEY, None) is None:
-                        request.session[LANGUAGE_SESSION_KEY] = unicode(browser_lang)
-                    break
+                    pass
+                elif browser_lang in prefixes:
+                    browser_lang = system_released_languages[prefixes.index(browser_lang)]
+                else:
+                    continue
+                if request.session.get(LANGUAGE_SESSION_KEY, None) is None:
+                            request.session[LANGUAGE_SESSION_KEY] = unicode(browser_lang)
+                break

openedx/core/djangoapps/lang_pref/tests/test_middleware.py

@@ -100,3 +100,30 @@ class TestUserPreferenceMiddleware(TestCase):
         set_user_preference(self.user, LANGUAGE_KEY, 'eo')
         self.middleware.process_request(self.request)
         self.assertEqual(get_user_preference(self.request.user, LANGUAGE_KEY), None)
+
+    @mock.patch(
+        'openedx.core.djangoapps.lang_pref.middleware.released_languages',
+        mock.Mock(return_value=[('eu-es', 'euskara (Espainia)'), ('en', 'english')])
+    )
+    def test_supported_browser_language_prefix_in_session(self):
+        """
+        test: browser language should be set in user session if it's prefix is supported by system for 
+        unathenticated users 
+        """
+        self.request.META['HTTP_ACCEPT_LANGUAGE'] = 'eu;q=1.0'
+        self.request.user = self.anonymous_user
+        self.middleware.process_request(self.request)
+        self.assertEqual(self.request.session.get(LANGUAGE_SESSION_KEY), 'eu-es')  #pylint: disable=no-member
+    
+    @mock.patch(
+        'openedx.core.djangoapps.lang_pref.middleware.released_languages',
+        mock.Mock(return_value=[('en', 'english')])
+    )
+    def test_unsupported_browser_language_prefix(self):
+        """
+        test: browser language should not be set in user session if it's prefix is not supported by system.
+        """
+        self.request.META['HTTP_ACCEPT_LANGUAGE']='eu;q=1.0'
+        self.request.user = self.anonymous_user
+        self.middleware.process_request(self.request)
+        self.assertNotEqual(self.request.session.get(LANGUAGE_SESSION_KEY), 'eu-es')   # pylint: disable=no-member