[1.8.x] Fixed #25295 -- Restored 'no active translation' after language override

Thanks David Nelson Adamec for the report and Tim Graham for the review.
Backport of 9324935c3 from master.

django/utils/translation/__init__.py

@@ -163,7 +163,9 @@ class override(ContextDecorator):
             deactivate_all()
 
     def __exit__(self, exc_type, exc_value, traceback):
-        if self.deactivate:
+        if self.old_language is None:
+            deactivate_all()
+        elif self.deactivate:
             deactivate()
         else:
             activate(self.old_language)

docs/releases/1.8.5.txt

@@ -13,3 +13,5 @@ Bugfixes
   field that is both a foreign and primary key (:ticket:`24951`).
 
 * Fixed a migrations crash with ``GenericForeignKey`` (:ticket:`25040`).
+* Made ``translation.override()`` clear the overridden language when a
+  translation isn't initially active (:ticket:`25295`).

tests/i18n/tests.py

@@ -74,6 +74,9 @@ class TranslationTests(TestCase):
             self.assertEqual(get_language(), 'de')
             with translation.override(None):
                 self.assertEqual(get_language(), None)
+                with translation.override('pl'):
+                    pass
+                self.assertEqual(get_language(), None)
             self.assertEqual(get_language(), 'de')
         finally:
             deactivate()