diff --git a/django/utils/translation/__init__.py b/django/utils/translation/__init__.py
index aa5cd33f5b..29ac60ad1d 100644
--- a/django/utils/translation/__init__.py
+++ b/django/utils/translation/__init__.py
@@ -201,9 +201,9 @@ def to_language(locale):
 
 def to_locale(language):
     """Turn a language name (en-us) into a locale name (en_US)."""
-    language, _, country = language.lower().partition('-')
+    lang, _, country = language.lower().partition('-')
     if not country:
-        return language
+        return language[:3].lower() + language[3:]
     # 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
@@ -212,7 +212,7 @@ def to_locale(language):
     country = country.title() if len(country) > 2 else country.upper()
     if tail:
         country += '-' + tail
-    return language + '_' + country
+    return lang + '_' + country
 
 
 def get_language_from_request(request, check_path=False):
diff --git a/tests/i18n/tests.py b/tests/i18n/tests.py
index 55cd35f7f3..5dd3593cac 100644
--- a/tests/i18n/tests.py
+++ b/tests/i18n/tests.py
@@ -313,12 +313,16 @@ class TranslationTests(SimpleTestCase):
             ('EN', 'en'),
             ('en-us', 'en_US'),
             ('EN-US', 'en_US'),
+            ('en_US', 'en_US'),
             # With > 2 characters after the dash.
             ('sr-latn', 'sr_Latn'),
             ('sr-LATN', 'sr_Latn'),
+            ('sr_Latn', 'sr_Latn'),
             # 3-char language codes.
             ('ber-MA', 'ber_MA'),
             ('BER-MA', 'ber_MA'),
+            ('BER_MA', 'ber_MA'),
+            ('ber_MA', 'ber_MA'),
             # With private use subtag (x-informal).
             ('nl-nl-x-informal', 'nl_NL-x-informal'),
             ('NL-NL-X-INFORMAL', 'nl_NL-x-informal'),