To be honest I do not know how what is the best practice haven I18n with CakePHP and JS. But I would add a JS code to the options like
    var options = {
      url: '$url',
      i18n = {
        notFound: '$notFound',
        anotherText: '$foo'
      }
    };

So that you can use options.i18n.notFound inside JS with localized strings. Keep in mind that the tick ' should be escaped.