I suggenst you create a http module and link it in web.config:

public class LangRewrite : IHttpModule
{
public void Init(System.Web.HttpApplication Appl)
{
//make sure to wire up to BeginRequest
Appl.BeginRequest += new System.EventHandler(Rewrite_BeginRequest);
}

public void Dispose()
{
}

public void Rewrite_BeginRequest(object sender, System.EventArgs args)
{
// here you can play with your request in any way you need and when you are done use HttpContext.Current.RewritePath(url) to rewrite the path internaly.
}
}

Then in web.config in httpmodules section add:

Hope this helped

One point to note… ASP.Net Ajax doesn’t like URL rewriting on .Net 2.0. If AJAX is needed on a .Net 2.0 site, you’ll need to write a custom HtmlTextWriter class to format the FORM action attribute appropriately. I imagine that these issues disappear with .Net 3.5 and IIS 7 with the built-in rewriting module and form.action, though I haven’t tested it extensively.