Index: src/wp-includes/l10n.php
===================================================================
--- src/wp-includes/l10n.php	(revision 38622)
+++ src/wp-includes/l10n.php	(working copy)
@@ -556,9 +556,28 @@
 	$mo = new MO();
 	if ( !$mo->import_from_file( $mofile ) ) return false;
 
-	if ( isset( $l10n[$domain] ) )
-		$mo->merge_with( $l10n[$domain] );
+	if ( isset( $l10n[$domain] ) ) {
 
+		$is_regular = false;
+		foreach ( array(
+			WP_LANG_DIR . '/plugins',
+			WP_LANG_DIR . '/themes',
+		) as $path ) {
+			$is_regular = strstr( $mofile, $path );
+
+			if ( $is_regular ) {
+				$is_regular = true;
+				break;
+			}
+		}
+
+		if ( ! $is_regular && 'default' !== $domain ) {
+			$mo->merge_originals_with( $l10n[$domain] );
+		} else {
+			$mo->merge_with( $l10n[$domain] );
+		}
+	}
+
 	unset( $l10n_unloaded[ $domain ] );
 
 	$l10n[$domain] = &$mo;