Localization in a web app using JavaScript and JSON
http://keith-wood.name/localisation.html
https://github.com/coderifous/jquery-localize
I don't have any personal experience in either but both appear to be fairly well documented.
Depending on your needs, the jquery-localize function might work well. If you need a more complex structure that mirrors that of your website, the localize library might be more useful: https://github.com/kflorence/localize