Index: wp-includes/l10n.php
===================================================================
--- wp-includes/l10n.php	(revision 8027)
+++ wp-includes/l10n.php	(working copy)
@@ -291,9 +291,11 @@
 	$locale = get_locale();
 
 	if ( false === $path )
-		$path = WP_PLUGIN_DIR;
+		$path = '';
+	else
+		$path = '/' . trim(trim($path), '/');
 
-	$mofile = $path . '/'. $domain . '-' . $locale . '.mo';
+	$mofile = WP_PLUGIN_DIR . $path . '/'. $domain . '-' . $locale . '.mo';
 	load_textdomain($domain, $mofile);
 }