Add a new getLanguage method to the various IL10n implementations

web/firefoxcom.js

@@ -145,6 +145,10 @@ class MozL10n {
     this.mozL10n = mozL10n;
   }
 
+  getLanguage() {
+    return Promise.resolve(this.mozL10n.getLanguage());
+  }
+
   getDirection() {
     return Promise.resolve(this.mozL10n.getDirection());
   }

web/genericl10n.js

@@ -27,6 +27,12 @@ class GenericL10n {
     });
   }
 
+  getLanguage() {
+    return this._ready.then((l10n) => {
+      return l10n.getLanguage();
+    });
+  }
+
   getDirection() {
     return this._ready.then((l10n) => {
       return l10n.getDirection();

web/interfaces.js

@@ -160,6 +160,11 @@ class IPDFAnnotationLayerFactory {
  * @interface
  */
 class IL10n {
+  /**
+   * @returns {Promise<string>} - Resolves to the current locale.
+   */
+  getLanguage() {}
+
   /**
    * @returns {Promise<string>} - Resolves to 'rtl' or 'ltr'.
    */

web/ui_utils.js

@@ -58,6 +58,10 @@ function formatL10nValue(text, args) {
  * @implements {IL10n}
  */
 let NullL10n = {
+  getLanguage() {
+    return Promise.resolve('en-us');
+  },
+
   getDirection() {
     return Promise.resolve('ltr');
   },