Index: wp-includes/general-template.php
===================================================================
--- wp-includes/general-template.php	(revision 7849)
+++ wp-includes/general-template.php	(working copy)
@@ -132,6 +132,8 @@
 			break;
 		case 'language':
 			$output = get_locale();
+			if (empty($output))
+				$output = 'en_US';
 			$output = str_replace('_', '-', $output);
 			break;
 		case 'text_direction':