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) 2006-2016 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 26 matching lines @@
       };
       document.addEventListener("DOMContentLoaded", onDOMContentLoaded);
     }
     else
     {
       setTimeout(callback, 0);
     }
   },
   get appLocale()
   {
-    var locale = ext.i18n.getMessage("@@ui_locale").replace(/_/g, "-");
+    var locale = ext.i18n.getMessage("@@ui_locale");
+    if (locale === "") {

[Sebastian Noack, 2016/03/14 09:03:06]

Nit: We always use == (instead ===) where possible. This is inline with
Mozilla's JavaScript coding style.

+        locale = ext.i18n.getUILanguage();
+    }
+    locale = locale.replace(/_/g, "-");
     Object.defineProperty(this, "appLocale", {value: locale, enumerable: true});
     return this.appLocale;
   },
+  get readingDirection()
+  {
+      var direction;
+      if ("chromeRegistry" in this) {
+        direction = this.chromeRegistry.isLocaleRTL("adblockplus") ? "rtl" : "ltr";

[Sebastian Noack, 2016/03/14 09:03:07]

This code path is specific to Adblock Plus for Firefox. The code here however is
only used on Chrome, Opera, Safari and Edge. So you can remove this branch.
However, you have to add Utils.readingDirection to the lib/utils.js in Adblock
Plus for Firefox as well.

+      }
+      else {      
+        var direction = ext.i18n.getMessage("@bidi_dir");
+        if (direction === "") {
+            direction = /^(ar|fa|he|ug|ur)(_|$)/.test(appLocale) ? "rtl" : "ltr";

[Sebastian Noack, 2016/03/14 09:03:07]

This logic is redundant with code in safari/ext/common.js which can be removed
now.

+        }
+      }
+      Object.defineProperty(this, "readingDirection", {value: direction, enumerable: true});
+      return this.readingDirection;
+  },
   generateChecksum: function(lines)
   {
     // We cannot calculate MD5 checksums yet :-(
     return null;
   },
   makeURI: function(url)
   {
     return Services.io.newURI(url);
   },
 
@@ skipping 53 matching lines @@
   {
     var Prefs = require("prefs").Prefs;
     var docLink = Prefs.documentation_link;
     return docLink.replace(/%LINK%/g, linkID).replace(/%LANG%/g, Utils.appLocale);
   },
 
   yield: function()
   {
   }
 };