It's extremely easy to enable this with basically one line of JavaScript (no jQuery).
This is the same form in two different languages:
http://s.codepen.io/figureone/debug/GgYXvE?#en-us
http://s.codepen.io/figureone/debug/GgYXvE?#en-gbThe language translations are all managed within the Form Editor on Marketo. In this example the form label Color/Colour is a rich text field using the standard HTML `lang` attribute to separate the languages:
<span lang="en-us">Favorite Color:</span>
<span lang="en-gb">Favourite Colour:</span>
Then CSS is used to display the one that matches the current language. In the demo, the language is passed in the #hashtag but it could come from anywhere.