diff --git a/CHANGES b/CHANGES
index 5b5039193..a958d185d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -114,6 +114,7 @@ Bugs fixed
 * #4152: HTML writer crashes if a field list is placed on top of the document
 * #4063: Sphinx crashes when labeling directive ``.. todolist::``
 * #4134: [doc] :file:`docutils.conf` is not documented explicitly
+* #4169: Chinese language doesn't trigger Chinese search automatically
 
 Testing
 --------
diff --git a/sphinx/ext/intersphinx.py b/sphinx/ext/intersphinx.py
index 73078330b..1ee58353c 100644
--- a/sphinx/ext/intersphinx.py
+++ b/sphinx/ext/intersphinx.py
@@ -217,7 +217,7 @@ def load_mappings(app):
 
         if isinstance(value, (list, tuple)):
             # new format
-            name, (uri, inv) = key, value  # type: ignore
+            name, (uri, inv) = key, value
             if not isinstance(name, string_types):
                 logger.warning('intersphinx identifier %r is not string. Ignored', name)
                 continue
diff --git a/sphinx/search/__init__.py b/sphinx/search/__init__.py
index 813a6dd51..a6074a863 100644
--- a/sphinx/search/__init__.py
+++ b/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):
diff --git a/tests/test_search.py b/tests/test_search.py
index 21c0badb7..f1825dfa4 100644
--- a/tests/test_search.py
+++ b/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'