Issue 3697 - Fall back to i18n.getUILanguage if @ui_locale isn't supported

lib/utils.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 25 matching lines @@
       };
       document.addEventListener("DOMContentLoaded", onDOMContentLoaded);
     }
     else
     {
       setTimeout(callback, 0);
     }
   },
   get appLocale()
   {
-    let locale = ext.i18n.getMessage("@@ui_locale").replace(/_/g, "-");
+    let locale = ext.i18n.getUILanguage();
     Object.defineProperty(this, "appLocale", {value: locale, enumerable: true});
     return this.appLocale;
   },
+  get readingDirection()
+  {
+    let direction = ext.i18n.getMessage("@@bidi_dir");
+    // This fallback is only necessary for Microsoft Edge
+    if (!direction)
+      direction = /^(?:ar|fa|he|ug|ur)\b/.test(this.appLocale) ? "rtl" : "ltr";
+    Object.defineProperty(
+      this,
+      "readingDirection",
+      {value: direction, enumerable: true}
+    );
+    return this.readingDirection;
+  },
   generateChecksum(lines)
   {
     // We cannot calculate MD5 checksums yet :-(
     return null;
   },
   checkLocalePrefixMatch(prefixes)
   {
     if (!prefixes)
       return null;
 
@@ skipping 50 matching lines @@
   {
     let docLink = require("prefs").Prefs.documentation_link;
     return docLink.replace(/%LINK%/g, linkID)
                   .replace(/%LANG%/g, Utils.appLocale);
   },
 
   yield()
   {
   }
 };