gistfile1.js

var jar = new CookieJar({path: '/', expires: 31536000}),
  // change these to match your site
  en = 'example.com/en', 
  fr = 'example.com/fr',
  // don't change this line or below
  matcher = new RegExp([en, '|', fr].join('').gsub('.','\\.'));

// did they already set a preference?
var preferred = jar.get('language');
if(!!preferred){
  if(!window.location.href.toString().include(preferred)){
    window.location.replace(window.location.href.toString().replace(matcher, preferred));
  }
}

// handle user input to change the language
document.on('click', '.language', function(evt, elm){
  if(elm.hasClassName('en'))
    jar.set('language',en);
  if(elm.hasClassName('fr'))
    jar.set('language',fr);
});