diff --git a/django/utils/translation/__init__.py b/django/utils/translation/__init__.py
index 28ab9c015f..7f5600be04 100644
--- a/django/utils/translation/__init__.py
+++ b/django/utils/translation/__init__.py
@@ -204,18 +204,18 @@ def to_language(locale):
 
 def to_locale(language):
     """Turn a language name (en-us) into a locale name (en_US)."""
-    language = language.lower()
-    parts = language.split('-')
-    try:
-        country = parts[1]
-    except IndexError:
+    language, _, country = language.lower().partition('-')
+    if not country:
         return language
     # A language with > 2 characters after the dash only has its first
     # character after the dash capitalized; e.g. sr-latn becomes sr_Latn.
     # A language with 2 characters after the dash has both characters
     # capitalized; e.g. en-us becomes en_US.
-    parts[1] = country.title() if len(country) > 2 else country.upper()
-    return parts[0] + '_' + '-'.join(parts[1:])
+    country, _, tail = country.partition('-')
+    country = country.title() if len(country) > 2 else country.upper()
+    if tail:
+        country += '-' + tail
+    return language + '_' + country
 
 
 def get_language_from_request(request, check_path=False):