diff --git a/django/utils/translation/trans_real.py b/django/utils/translation/trans_real.py
index 6b3aeb127e..a9447d7896 100644
--- a/django/utils/translation/trans_real.py
+++ b/django/utils/translation/trans_real.py
@@ -56,20 +56,14 @@ def reset_cache(**kwargs):
         get_supported_language_variant.cache_clear()
 
 
-def to_locale(language, to_lower=False):
-    """
-    Turn a language name (en-us) into a locale name (en_US). If 'to_lower' is
-    True, the last component is lower-cased (en_us).
-    """
+def to_locale(language):
+    """Turn a language name (en-us) into a locale name (en_US)."""
     p = language.find('-')
     if p >= 0:
-        if to_lower:
-            return language[:p].lower() + '_' + language[p + 1:].lower()
-        else:
-            # Get correct locale for sr-latn
-            if len(language[p + 1:]) > 2:
-                return language[:p].lower() + '_' + language[p + 1].upper() + language[p + 2:].lower()
-            return language[:p].lower() + '_' + language[p + 1:].upper()
+        # Get correct locale for sr-latn
+        if len(language[p + 1:]) > 2:
+            return language[:p].lower() + '_' + language[p + 1].upper() + language[p + 2:].lower()
+        return language[:p].lower() + '_' + language[p + 1:].upper()
     else:
         return language.lower()