function getUrlVars()
{
   var map = {};
   var parts = window.location.href.replace(/[?&]+([^=&]+)=([^&]*)/gi, function(m,key,value) {
      map[key] = value;
   });
   return map;
}

var urlVars = getUrlVars(); //call function
document.write(urlVars["lang"]); //this will write "en" to the document as in the above example

CKEDITOR.replace("editor1",
{
     language: urlVars["lang"]
});

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script src="ckeditor/ckeditor.js" type="text/javascript"></script>
</head>
<body>
    <textarea name="editor1"></textarea>
    <script type="text/javascript">
        window.onload = function ()
        {
            function getUrlVars()
            {
                var map = {};
                var parts = window.location.href.replace(/[?&]+([^=&]+)=([^&]*)/gi, function (m, key, value)
                {
                    map[key] = value;
                });
                return map;
            }

            var urlVars = getUrlVars(); //get the lang from the GET variable "lang"

            CKEDITOR.replace("editor1",
            {
                language: urlVars["lang"]
            });
        };
    </script>
</body>
</html>