Add 'vary: accept-language' header to locale choice reponse

lib/l10n_utils/__init__.py

@@ -87,7 +87,10 @@ def locale_selection(request, available_locales=None):
         "languages": product_details.languages,
         "available_locales": sorted(set(available_locales)),
     }
-    return django_render(request, "404-locale.html", context, status=200 if is_root else 404)
+    response = django_render(request, "404-locale.html", context, status=200 if is_root else 404)
+    # Add the Vary header to avoid improper cache
+    response["Vary"] = "Accept-Language"
+    return response
 
 
 def render(request, template, context=None, ftl_files=None, activation_files=None, **kwargs):

lib/l10n_utils/tests/test_base.py

@@ -51,6 +51,8 @@ class TestRender(TestCase):
             self.assertEqual(response.status_code, 404)
         else:
             self.assertEqual(response.status_code, 200)
+            if path == "/":
+                self.assertEqual(response["Vary"], "Accept-Language")
 
     def test_no_accept_language_header(self):
         template = "firefox/new.html"