Index: wp-includes/update.php
===================================================================
--- wp-includes/update.php	(revision 15591)
+++ wp-includes/update.php	(working copy)
@@ -33,7 +33,7 @@
 		$current->version_checked = $wp_version;
 	}
 
-	$locale = apply_filters( 'core_version_check_locale', get_locale() );
+	$locale = apply_filters( 'core_version_check_locale', get_locale( true ) );
 
 	// Update last_checked for current to prevent multiple blocking requests if request hangs
 	$current->last_checked = time();
Index: wp-includes/l10n.php
===================================================================
--- wp-includes/l10n.php	(revision 15590)
+++ wp-includes/l10n.php	(working copy)
@@ -23,9 +23,11 @@
  * @uses apply_filters() Calls 'locale' hook on locale value.
  * @uses $locale Gets the locale stored in the global.
  *
+ * @param bool $network Whether to retrieve the network wide setting when running
+ * 	multisite. Optional, default false.
  * @return string The locale of the blog or from the 'locale' hook.
  */
-function get_locale() {
+function get_locale( $network = false ) {
 	global $locale;
 
 	if ( isset( $locale ) )
@@ -38,7 +40,7 @@
 	// If multisite, check options.
 	if ( is_multisite() && !defined('WP_INSTALLING') ) {
 		$ms_locale = get_option('WPLANG');
-		if ( $ms_locale === false )
+		if ( $network || false === $ms_locale )
 			$ms_locale = get_site_option('WPLANG');
 
 		if ( $ms_locale !== false )