Index: wp-includes/l10n.php
===================================================================
--- wp-includes/l10n.php	(revision 17165)
+++ wp-includes/l10n.php	(working copy)
@@ -32,8 +32,11 @@
 		return apply_filters( 'locale', $locale );
 
 	// WPLANG is defined in wp-config.
-	if ( defined( 'WPLANG' ) )
+	if ( defined( 'WPLANG' ) ) {
 		$locale = WPLANG;
+		if ( 2 == strlen( $locale ) )
+			$locale = strtolower( $locale ) . '_' . strtoupper( $locale );
+	}
 
 	// If multisite, check options.
 	if ( is_multisite() && !defined('WP_INSTALLING') ) {