Fix #4169: Chinese language doesn't trigger Chinese search automatically

CHANGES

@@ -31,6 +31,7 @@ Bugs fixed
 * #3692: Unable to build HTML if writing .buildinfo failed
 * #4152: HTML writer crashes if a field list is placed on top of the document
 * #4063: Sphinx crashes when labeling directive ``.. todolist::``
+* #4169: Chinese language doesn't trigger Chinese search automatically
 
 Testing
 --------

sphinx/search/__init__.py

@@ -265,6 +265,11 @@ class IndexBuilder(object):
                                     # objtype index -> (domain, type, objname (localized))
         lang_class = languages.get(lang)    # type: Type[SearchLanguage]
                                             # add language-specific SearchLanguage instance
+
+        # fallback; try again with language-code
+        if lang_class is None and '_' in lang:
+            lang_class = languages.get(lang.split('_')[0])
+
         if lang_class is None:
             self.lang = SearchEnglish(options)  # type: SearchLanguage
         elif isinstance(lang_class, str):

tests/test_search.py

@@ -228,3 +228,15 @@ def test_IndexBuilder():
     }
     assert index._objtypes == {('dummy', 'objtype'): 0}
     assert index._objnames == {0: ('dummy', 'objtype', 'objtype')}
+
+
+def test_IndexBuilder_lookup():
+    env = DummyEnvironment('1.0', {})
+
+    # zh
+    index = IndexBuilder(env, 'zh', {}, None)
+    assert index.lang.lang == 'zh'
+
+    # zh_CN
+    index = IndexBuilder(env, 'zh_CN', {}, None)
+    assert index.lang.lang == 'zh'