I have a hack that I think uses very little code and is quite reliable.
Put your site's files in a subdirectory. SSL into your server and create symlinks to that subdirectory where your files are stored that indicate your languages.
Something like this:
ln -s /var/www/yourhtml /var/www/en
ln -s /var/www/yourhtml /var/www/sp
ln -s /var/www/yourhtml /var/www/it
Use your web server to read HTTP_ACCEPT_LANGUAGE and redirect to these "different subdirectories" according to the language value it provides.
Now you can use Javascript's window.location.href to get your url and use it in conditionals to reliably identify the preferred language.
url_string = window.location.href;
if (url_string = "http://yoursite.com/it/index.html") {
document.getElementById("page-wrapper").className = "italian";
}