Use default language if none match accept-language

Fix bug 1752823

lib/l10n_utils/__init__.py

@@ -170,6 +170,10 @@ def get_best_translation(translations, accept_languages):
 
     """
     lang_map = _get_language_map()
+    # translations contains mixed-case items e.g. "en-US" while the keys
+    # of `lang_map` are all lowercase. But this works because the values
+    # of the `lang_map` dict are mixed-case like translations and the
+    # comparison below is with the values.
     valid_lang_map = {k: v for k, v in lang_map.items() if v in translations}
     for lang in accept_languages:
         lang.lower()
@@ -179,7 +183,14 @@ def get_best_translation(translations, accept_languages):
         if pre in valid_lang_map:
             return valid_lang_map[pre]
 
-    return lang_map[translations[0].lower()]
+    # If all the attempts failed, just use en-US, the default locale of
+    # the site, if it is an available translation.
+    if settings.LANGUAGE_CODE in translations:
+        return settings.LANGUAGE_CODE
+
+    # In the rare case the default language isn't in the list, return the
+    # first translation in the valid_lang_map.
+    return list(valid_lang_map.values())[0]
 
 
 def get_translations_native_names(locales):

lib/l10n_utils/tests/test_base.py

@@ -3,7 +3,7 @@
 # file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 import os
-from unittest.mock import ANY, call, patch
+from unittest.mock import ANY, Mock, call, patch
 
 from django.test import TestCase
 from django.test.client import RequestFactory
@@ -168,6 +168,7 @@ def test_ftl_activations(self, render_mock):
         render_mock.assert_called_with(self.req, ["dude.html"], ANY, ftl_files="dude", activation_files=["dude", "donny"])
 
 
+@patch.object(l10n_utils, "_get_language_map", Mock(return_value={"an": "an", "de": "de", "en": "en-US", "en-us": "en-US", "fr": "fr"}))
 @pytest.mark.parametrize(
     "translations, accept_languages, expected",
     (
@@ -184,6 +185,10 @@ def test_ftl_activations(self, render_mock):
         (["en-US", "de"], ["de", "en-US"], "de"),
         (["en-US", "de", "fr"], ["fr", "de"], "fr"),
         (["en-US", "de", "fr"], ["en-CA", "fr", "de"], "en-US"),
+        # A request for only inactive translations should default to 'en-US'. Bug 1752823
+        (["am", "an", "en-US"], ["mk", "gu-IN"], "en-US"),
+        # "am" is not a valid language in the list of PROD_LANGUAGES
+        (["am", "an"], ["mk", "gu-IN"], "an"),
     ),
 )
 def test_get_best_translation(translations, accept_languages, expected):