Allow released languages to be previewed under dark lang

common/djangoapps/dark_lang/middleware.py

@@ -126,7 +126,4 @@ class DarkLangMiddleware(object):
         if not preview_lang:
             return
 
-        if preview_lang in self.released_langs:
-            return
-
         request.session['django_language'] = preview_lang

common/djangoapps/dark_lang/tests.py

@@ -155,13 +155,14 @@ class DarkLangMiddlewareTests(TestCase):
         )
 
     def test_preview_lang_with_released_language(self):
+        # Preview lang should always override selection.
         self.assertSessionLangEquals(
-            UNSET,
+            'rel',
             self.process_request(preview_lang='rel')
         )
 
         self.assertSessionLangEquals(
-            'notrel',
+            'rel',
             self.process_request(preview_lang='rel', django_language='notrel')
         )