[3.0.x] Fixed #31141 -- Relaxed system check of translation settings for sublanguages.

Regression in 4400d8296d268f5a8523cd02ddc33b12219b2535.

Thanks Enrique Matías Sánchez for the report.
Backport of 53d8646f799de7f92ab9defe9dc56c6125448102 from master

3 files changed, 39 insertions, 8 deletions

diff --git a/django/core/checks/translation.py b/django/core/checks/translation.py
index 138ed07ff8..8457a6b89d 100644
--- a/django/core/checks/translation.py
+++ b/django/core/checks/translation.py
@@ -1,4 +1,5 @@
 from django.conf import settings
+from django.utils.translation import get_supported_language_variant
 from django.utils.translation.trans_real import language_code_re
 
 from . import Error, Tags, register
@@ -55,7 +56,9 @@ def check_setting_languages_bidi(app_configs, **kwargs):
 @register(Tags.translation)
 def check_language_settings_consistent(app_configs, **kwargs):
     """Error if language settings are not consistent with each other."""
-    available_tags = {i for i, _ in settings.LANGUAGES} | {'en-us'}
-    if settings.LANGUAGE_CODE not in available_tags:
+    try:
+        get_supported_language_variant(settings.LANGUAGE_CODE)
+    except LookupError:
         return [E004]
-    return []
+    else:
+        return []
diff --git a/docs/releases/3.0.3.txt b/docs/releases/3.0.3.txt
index 61ef456d93..27ace475d1 100644
--- a/docs/releases/3.0.3.txt
+++ b/docs/releases/3.0.3.txt
@@ -16,3 +16,7 @@ Bugfixes
 * Fixed a regression in Django 3.0 where ``QuerySet.values()`` and
   ``values_list()`` crashed if a queryset contained an aggregation and
   ``Exists()`` annotation (:ticket:`31136`).
+
+* Relaxed the system check added in Django 3.0 to reallow use of a sublanguage
+  in the :setting:`LANGUAGE_CODE` setting, when a base language is available in
+  Django but the sublanguage is not (:ticket:`31141`).
diff --git a/tests/check_framework/test_translation.py b/tests/check_framework/test_translation.py
index 4cd79d803f..8747a52cda 100644
--- a/tests/check_framework/test_translation.py
+++ b/tests/check_framework/test_translation.py
@@ -3,7 +3,7 @@ from django.core.checks.translation import (
     check_language_settings_consistent, check_setting_language_code,
     check_setting_languages, check_setting_languages_bidi,
 )
-from django.test import SimpleTestCase
+from django.test import SimpleTestCase, override_settings
 
 
 class TranslationCheckTests(SimpleTestCase):
@@ -75,12 +75,36 @@ class TranslationCheckTests(SimpleTestCase):
                     Error(msg % tag, id='translation.E003'),
                 ])
 
+    @override_settings(USE_I18N=True, LANGUAGES=[('en', 'English')])
     def test_inconsistent_language_settings(self):
         msg = (
             'You have provided a value for the LANGUAGE_CODE setting that is '
             'not in the LANGUAGES setting.'
         )
-        with self.settings(LANGUAGE_CODE='fr', LANGUAGES=[('en', 'English')]):
-            self.assertEqual(check_language_settings_consistent(None), [
-                Error(msg, id='translation.E004'),
-            ])
+        for tag in ['fr', 'fr-CA', 'fr-357']:
+            with self.subTest(tag), self.settings(LANGUAGE_CODE=tag):
+                self.assertEqual(check_language_settings_consistent(None), [
+                    Error(msg, id='translation.E004'),
+                ])
+
+    @override_settings(
+        USE_I18N=True,
+        LANGUAGES=[
+            ('de', 'German'),
+            ('es', 'Spanish'),
+            ('fr', 'French'),
+            ('ca', 'Catalan'),
+        ],
+    )
+    def test_valid_variant_consistent_language_settings(self):
+        tests = [
+            # language + region.
+            'fr-CA',
+            'es-419',
+            'de-at',
+            # language + region + variant.
+            'ca-ES-valencia',
+        ]
+        for tag in tests:
+            with self.subTest(tag), self.settings(LANGUAGE_CODE=tag):
+                self.assertEqual(check_language_settings_consistent(None), [])