Fixed #32479 -- Added fallbacks to subsequent language codes in translations.

Thanks Claude Paroz and Nick Pope for reviews.

django/utils/translation/trans_real.py

@@ -474,14 +474,17 @@ def get_supported_language_variant(lang_code, strict=False):
     <https://www.djangoproject.com/weblog/2007/oct/26/security-fix/>.
     """
     if lang_code:
-        # If 'fr-ca' is not supported, try special fallback or language-only 'fr'.
+        # If 'zh-hant-tw' is not supported, try special fallback or subsequent
+        # language codes i.e. 'zh-hant' and 'zh'.
         possible_lang_codes = [lang_code]
         try:
             possible_lang_codes.extend(LANG_INFO[lang_code]['fallback'])
         except KeyError:
             pass
-        generic_lang_code = lang_code.split('-')[0]
-        possible_lang_codes.append(generic_lang_code)
+        i = None
+        while (i := lang_code.rfind('-', 0, i)) > -1:
+            possible_lang_codes.append(lang_code[:i])
+        generic_lang_code = possible_lang_codes[-1]
         supported_lang_codes = get_languages()
 
         for code in possible_lang_codes:

tests/i18n/tests.py

@@ -1424,6 +1424,26 @@ class MiscTests(SimpleTestCase):
         r.META = {'HTTP_ACCEPT_LANGUAGE': 'zh-my,en'}
         self.assertEqual(get_language_from_request(r), 'zh-hans')
 
+    def test_subsequent_code_fallback_language(self):
+        """
+        Subsequent language codes should be used when the language code is not
+        supported.
+        """
+        tests = [
+            ('zh-Hans-CN', 'zh-hans'),
+            ('zh-hans-mo', 'zh-hans'),
+            ('zh-hans-HK', 'zh-hans'),
+            ('zh-Hant-HK', 'zh-hant'),
+            ('zh-hant-tw', 'zh-hant'),
+            ('zh-hant-SG', 'zh-hant'),
+        ]
+        r = self.rf.get('/')
+        r.COOKIES = {}
+        for value, expected in tests:
+            with self.subTest(value=value):
+                r.META = {'HTTP_ACCEPT_LANGUAGE': f'{value},en'}
+                self.assertEqual(get_language_from_request(r), expected)
+
     def test_parse_language_cookie(self):
         """
         Now test that we parse language preferences stored in a cookie correctly.