As far as I know, there’s a good library called localeplanet for Localization and Internationalization in JavaScript. Furthermore, I think it’s native and has no dependencies to other libraries (e.g. jQuery)
Here’s the website of library: http://www.localeplanet.com/
Also look at this article by Mozilla, you can find very good method and algorithms for client-side translation: http://blog.mozilla.org/webdev/2011/10/06/i18njs-internationalize-your-javascript-with-a-little-help-from-json-and-the-server/
The common part of all those articles/libraries is that they use a i18n class and a get method (in some ways also defining an smaller function name like _) for retrieving/converting the key to the value. In my explaining the key means that string you want to translate and the value means translated string.
Then, you just need a JSON document to store key‘s and value‘s.
For example:
var _ = document.webL10n.get;
alert(_('test'));
And here the JSON:
{ test: "blah blah" }
I believe using current popular libraries solutions is a good approach.