[1.8.x] Fixed #24413 -- Prevented translation fallback for English

Thanks Tomasz Kontusz for the report, Baptiste Mispelon for
analysis and Tim Graham for the review.
Backport of 3cf1c02695 from master.

django/utils/translation/trans_real.py

@@ -176,11 +176,9 @@ class DjangoTranslation(gettext_module.GNUTranslations):
 
     def _add_fallback(self):
         """Sets the GNUTranslations() fallback with the default language."""
-        # Don't set a fallback for the default language or for
-        # en-us (as it's empty, so it'll ALWAYS fall back to the default
-        # language; found this as part of #21498, as we set en-us for
-        # management commands)
-        if self.__language == settings.LANGUAGE_CODE or self.__language == "en-us":
+        # Don't set a fallback for the default language or any English variant
+        # (as it's empty, so it'll ALWAYS fall back to the default language)
+        if self.__language == settings.LANGUAGE_CODE or self.__language.startswith('en'):
             return
         default_translation = translation(settings.LANGUAGE_CODE)
         self.add_fallback(default_translation)

tests/i18n/tests.py

@@ -901,6 +901,21 @@ class MiscTests(TestCase):
         super(MiscTests, self).setUp()
         self.rf = RequestFactory()
 
+    @override_settings(LANGUAGE_CODE='de')
+    def test_english_fallback(self):
+        """
+        With a non-English LANGUAGE_CODE and if the active language is English
+        or one of its variants, the untranslated string should be returned
+        (instead of falling back to LANGUAGE_CODE) (See #24413).
+        """
+        self.assertEqual(ugettext("Image"), "Bild")
+        with translation.override('en'):
+            self.assertEqual(ugettext("Image"), "Image")
+        with translation.override('en-us'):
+            self.assertEqual(ugettext("Image"), "Image")
+        with translation.override('en-ca'):
+            self.assertEqual(ugettext("Image"), "Image")
+
     def test_parse_spec_http_header(self):
         """
         Testing HTTP header parsing. First, we test that we can parse the