Index: config/main.inc.php.dist
===================================================================
--- config/main.inc.php.dist	(revision d224551cfbf5903505f41bc9e51ad9840119942f)
+++ config/main.inc.php.dist	(revision 522007bdb26b737cb4a7066618a2e877211be9d0)
@@ -120,5 +120,5 @@
 
 // the default locale setting
-$rcmail_config['language'] = 'en';
+$rcmail_config['language'] = 'en_US';
 
 // use this format for short date display