There is no decent way to get that setting, at least not something browser independent.
But the server has that info, because it is part of the HTTP request header (the Accept-Language field, see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4)
So the only reliable way is to get an answer back from the server. You will need something that runs on the server (like .asp, .jsp, .php, CGI) and that "thing" can return that info.
Good examples here: http://www.developershome.com/wap/detection/detection.asp?page=readHeader