Monkeypatch Django's check_for_language() because it looks in the wro…

…ng place for us

By default check_for_language() seeks gettext-compatible files in locale/ directories
in the project, but we don't have those because our l10n is handled using Fluent
strings in the www-l10n-team directory, cloned from their git repo.

So, instead, we just perform a light check to see whether a given lang is among the
langs that shold be available. The actual decision about whether we have have (enough)
Fluent strings available for that locale happens in lib.l10n_utils.render()

bedrock/base/i18n.py

@@ -159,3 +159,7 @@ def get_best_language(accept_lang):
         supported = normalize_language(lang)
         if supported:
             return supported
+
+
+def check_for_bedrock_language(lang_code):
+    return lang_code in [x[0] for x in settings.LANGUAGES]

bedrock/base/middleware.py

@@ -21,11 +21,13 @@
 from django.middleware.locale import LocaleMiddleware as DjangoLocaleMiddleware
 from django.utils import translation
 from django.utils.deprecation import MiddlewareMixin
+from django.utils.translation import trans_real
 
 from commonware.middleware import FrameOptionsHeader as OldFrameOptionsHeader
 
 from bedrock.base import metrics
 from bedrock.base.i18n import (
+    check_for_bedrock_language,
     get_language_from_headers,
     normalize_language,
     path_needs_lang_code,
@@ -98,11 +100,13 @@ class BedrockLocaleMiddleware(DjangoLocaleMiddleware):
 
     def process_request(self, request):
         with normalized_get_language():
-            return super().process_request(request)
+            with simplified_check_for_language():
+                return super().process_request(request)
 
     def process_response(self, request, response):
         with normalized_get_language():
-            return super().process_response(request, response)
+            with simplified_check_for_language():
+                return super().process_response(request, response)
 
 
 @contextlib.contextmanager
@@ -132,6 +136,26 @@ def get_normalized_language():
         translation.get_language = get_language
 
 
+@contextlib.contextmanager
+def simplified_check_for_language():
+    """Ensures that calls to trans_real.check_for_language within
+    its context will not check for the existence of files in
+    `appname/LANG_CODE/locale` and therefore avoid a false negative about
+    the langs we support (our lang files are in ./data/l10n-team/LANG_CODE)
+    but check_for_language is opinionated and expects only the Django directory
+    pattern for lang files."""
+
+    check_for_language = trans_real.check_for_language
+
+    @wraps(check_for_language)
+    def simpler_check_for_language(lang_code):
+        return check_for_bedrock_language(lang_code)
+
+    trans_real.check_for_language = simpler_check_for_language
+
+    yield
+
+
 class BasicAuthMiddleware:
     """
     Middleware to protect the entire site with a single basic-auth username and password.

bedrock/base/tests/test_i18n.py

@@ -8,6 +8,7 @@
 
 from bedrock.base.i18n import (
     LocalePrefixPattern,
+    check_for_bedrock_language,
     normalize_language,
     path_needs_lang_code,
     split_path_and_polish_lang,
@@ -226,3 +227,26 @@ def test_locale_prefix_pattern_works(language_to_activate, expected_prefix):
 
     assert pattern.language_prefix == expected_prefix
     translation.deactivate()
+
+
+@override_settings(
+    LANGUAGES=[
+        ("en-US", "English"),
+        ("fr", "French"),
+        ("sco", "Scots"),
+        ("hsb", "Hornjoserbsce"),
+    ]
+)
+@pytest.mark.parametrize(
+    "lang_code, expected_result",
+    (
+        ("en-US", True),
+        ("fr", True),
+        ("sco", True),
+        ("hsb", True),
+        ("ach", False),
+        ("de", False),
+    ),
+)
+def test_check_for_bedrock_language(lang_code, expected_result):
+    assert check_for_bedrock_language(lang_code) == expected_result