diff --git a/tornado/locale.py b/tornado/locale.py
index 52aa1af9d5..aea9e1f3bb 100644
--- a/tornado/locale.py
+++ b/tornado/locale.py
@@ -248,6 +248,10 @@ def get_closest(cls, *locale_codes: str) -> "Locale":
                 return cls.get(code)
             if parts[0].lower() in _supported_locales:
                 return cls.get(parts[0].lower())
+            if len(parts) == 1:
+                for supported in _supported_locales:
+                    if parts[0].lower() == supported.split("_")[0]:
+                        return cls.get(supported)
         return cls.get(_default_locale)
 
     @classmethod