DanSingerman has a very good solution for this question.
The only reliable source for the language is in the HTTP-request header.
So you need a server-side script to reply the request-header or at least the Accept-Language
field back to you.
Here is a very simple Node.js server which should be compatible with DanSingermans jQuery plugin.
var http = require('http');
http.createServer(function (req, res) {
res.writeHead(200, {'Content-Type': 'text/plain'});
res.end(JSON.stringify(req.headers));
}).listen(80,'0.0.0.0');