Dynamically switching language PHP,Javascript, jQuery UI

I would do this way, have a separate PHP file responsible for getting correct JS File, this way you can use it in plan HTML pages as well...

<script type="text/javascript" src="/getCorrectJsFile.php"></script>

What do you think of this approach?

function getBaseURL()
{
    var url = location.href;
    var baseURL = url.substring(0, url.indexOf('/', 14));
    if (baseURL.indexOf('http://localhost') != -1)
    {
        var url = location.href; 
        var pathname = location.pathname;  
        var index1 = url.indexOf(pathname);
        var index2 = url.indexOf("/", index1 + 1);
        var baseLocalUrl = url.substr(0, index2);
        return baseLocalUrl + "/";
    }
    else
    {
        return baseURL + "/";
    }
}

function getCookie(c_name)
{
    var i,x,y,ARRcookies=document.cookie.split(";");
    for (i=0;i<ARRcookies.length;i++)
    {
        x=ARRcookies[i].substr(0,ARRcookies[i].indexOf("="));
        y=ARRcookies[i].substr(ARRcookies[i].indexOf("=")+1);
        x=x.replace(/^\s+|\s+$/g,"");
        if (x==c_name)
        {
            return unescape(y);
        }
   }
}


function getLang()
{
    var lang = getCookie("lang");
    switch(lang)
    {
        case "fr":
            document.write("<script src='"+getBaseURL()+"js/lang/jquery.ui.datepicker-"+lang+".js'></script>");
            break;
        default:
            document.write("");
    }
}
getLang();

The functions would be located in a common js file and I would call the getLang() function on all of my pages...

How about using .change() and .post()? You add id of lang to your form where a user can change the language.

$('#lang').change(function(){
           //link to your php file
    var findlangfile = "<?php echo index.php/findlang.php;?>
    $.post(findlangfile, {
        lang: $('#lang').val(),
    }, function(response){
        // you do things
    });
    return false;
});

You need to put a check in your header.php which will goes like that:

<script type="text/javascript>" src="/js/<?php if($_SESSION['lang'] == "en"){ echo 'datepicker-en.js'}else echo 'datepicker-fr.js'; ?>"></script>

So it will automatically detect the datepicker file according to your language parameter.

You could use jQuery cookie plugin, a good one. Then do same procedure with jQuery as with php. If language supplied as url parameter - set cookie with the plugin (get language value with the getBaseURL() function above). Otherwise load cookie and then load appropriate javascript file. Something like $('head').append('<script..'); should do the task.

Make sure to run this whole script on page load with $(function() { /* code here */ });

Although it's possible to accomplish the task this way, I would highly recommend switching all your pages to php, as it would simplify things like this one.