Index: wp-includes/l10n.php
===================================================================
--- wp-includes/l10n.php	(revision 18776)
+++ wp-includes/l10n.php	(working copy)
@@ -36,9 +36,8 @@
 		$locale = WPLANG;
 
 	// If multisite, check options.
-	if ( is_multisite() && !defined('WP_INSTALLING') ) {
-		$ms_locale = get_option('WPLANG');
-		if ( $ms_locale === false )
+	if ( is_multisite() ) {
+		if ( defined( 'WP_INSTALLING' ) || false === ( $ms_locale = get_option( 'WPLANG' ) ) )
 			$ms_locale = get_site_option('WPLANG');
 
 		if ( $ms_locale !== false )