--- a/intl/l10n/DOMLocalization.jsm
+++ b/intl/l10n/DOMLocalization.jsm
@@ -15,16 +15,18 @@
  * limitations under the License.
  */
 
 
 /* fluent-dom@aa95b1f (July 10, 2018) */
 
 const { Localization } =
   ChromeUtils.import("resource://gre/modules/Localization.jsm", {});
+const { Services } =
+  ChromeUtils.import("resource://gre/modules/Services.jsm", {});
 
 // Match the opening angle bracket (<) in HTML tags, and HTML entities like
 // &amp;, &#0038;, &#x0026;.
 const reOverlay = /<|&#?\w+;/;
 
 /**
  * Elements allowed in translations even if they are not present in the source
  * HTML. They are text-level elements as defined by the HTML5 spec:
@@ -567,17 +569,24 @@ class DOMLocalization extends Localizati
   /**
    * Translate all roots associated with this `DOMLocalization`.
    *
    * @returns {Promise}
    */
   translateRoots() {
     const roots = Array.from(this.roots);
     return Promise.all(
-      roots.map(root => this.translateFragment(root))
+      roots.map(async root => {
+        await this.translateFragment(root);
+        let primaryLocale = Services.locale.getAppLocaleAsBCP47();
+        let direction = Services.locale.isAppLocaleRTL ? "rtl" : "ltr";
+        root.setAttribute("lang", primaryLocale);
+        root.setAttribute(
+          root.namespaceURI === "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" ? "localedir" : "dir", direction);
+      })
     );
   }
 
   /**
    * Pauses the `MutationObserver`.
    *
    * @private
    */