Fixed #18149 -- Changed language codes for Chinese

Language codes for Chinese are zh_Hans (Simplified) and zh_Hant (Traditional).
Added support for browsers that still send the deprecated language codes.

Thanks to Olli Wang for the report.

tests/deprecation/tests.py

@@ -1,8 +1,8 @@
 from __future__ import unicode_literals
 import warnings
 
-from django.test import SimpleTestCase, RequestFactory
-from django.utils import six
+from django.test import SimpleTestCase, RequestFactory, override_settings
+from django.utils import six, translation
 from django.utils.deprecation import RenameMethodsBase
 
 
@@ -186,3 +186,22 @@ class DeprecatingRequestMergeDictTest(SimpleTestCase):
                 '`request.POST` instead.',
                 '`MergeDict` is deprecated, use `dict.update()` instead.',
             ])
+
+
+@override_settings(USE_I18N=True)
+class DeprecatedChineseLanguageCodes(SimpleTestCase):
+    def test_deprecation_warning(self):
+        warnings.simplefilter('always')
+
+        with warnings.catch_warnings(record=True) as recorded:
+            with translation.override('zh-cn'):
+                pass
+            with translation.override('zh-tw'):
+                pass
+            msgs = [str(warning.message) for warning in recorded]
+            self.assertEqual(msgs, [
+                "The use of the language code 'zh-cn' is deprecated. "
+                "Please use the 'zh-hans' translation instead.",
+                "The use of the language code 'zh-tw' is deprecated. "
+                "Please use the 'zh-hant' translation instead.",
+            ])