Non-standard
 
      This feature is not on a current W3C standards track, but it is supported on the Firefox OS platform. Although implementations may change in the future and it is not supported widely across browsers, it is suitable for use in code dedicated to Firefox OS apps.
The language.direction property returns the direction (ltr or rtl) of the currently active language.
Syntax
var languageCode = navigator.mozL10n.language.direction;
Value
Returns the direction of the currently active language (ltr or rtl).
Example
navigator.mozL10n.ready(function() {
  console.log('The current language's direction is ' + navigator.mozL10n.language.direction);
});
Specification
Not part of any specification.