--- modules\locale\locale.module
+++ modules\locale\locale.module
@@ -548,10 +549,30 @@
 
   // Add the translation JavaScript file to the page.
   if ($files && !empty($language->javascript)) {
+    switch (variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC)) {
+      case FILE_DOWNLOADS_PUBLIC:
     drupal_add_js($dir .'/'. $language->language .'_'. $language->javascript .'.js', 'core');
+        return;
+      case FILE_DOWNLOADS_PRIVATE:
+        drupal_add_js('system/files/' . variable_get('locale_js_directory', 'languages') . '/' . $language->language . '_' . $language->javascript . '.js', 'core');
+        return;
   }
 }
+}
 
+/**
+ * Implementation of hook_file_download()
+ * Tell Drupal that we need to access Javascript files in this directory if private download is enabled.
+ */
+function locale_file_download($filepath) {
+    if (strpos($filepath, variable_get('locale_js_directory', 'languages')) === 0) {
+        $mime = file_get_mimetype($filepath);
+        if (strpos($mime, 'javascript') !== false) {
+            return array('Content-type:' . $mime);
+        }
+    }
+}
+
 // ---------------------------------------------------------------------------------
 // Language switcher block