Fixed #21473 -- Limited language preservation to logout

Current language is no longer saved to session by LocaleMiddleware
on  every response (the behavior introduced in #14825).
Instead language stored in session is reintroduced into new session
after logout.

Forward port of c558a43fd6 to master.

django/contrib/auth/tests/test_views.py

@@ -1,3 +1,4 @@
+from importlib import import_module
 import itertools
 import os
 import re
@@ -710,6 +711,18 @@ class LogoutTest(AuthViewsTestCase):
                             "%s should be allowed" % good_url)
             self.confirm_logged_out()
 
+    def test_logout_preserve_language(self):
+        """Check that language stored in session is preserved after logout"""
+        # Create a new session with language
+        engine = import_module(settings.SESSION_ENGINE)
+        session = engine.SessionStore()
+        session['_language'] = 'pl'
+        session.save()
+        self.client.cookies[settings.SESSION_COOKIE_NAME] = session.session_key
+
+        self.client.get('/logout/')
+        self.assertEqual(self.client.session['_language'], 'pl')
+
 
 @skipIfCustomUser
 @override_settings(