You can use a virtual directory in the iis and point it to an common folder for all langauge depend pages.

e.g
/en/yourPage.aspx
/es/yourPage.aspx

the directory structur could be :

\bin
\common\yourPage.aspx
\controls\.....

to switch the language read the current page, create some links to the current page and prepend the
different langauge codes to the links.

- for a first-time visitor, read whose language from the request
- to redirect a visitor to the last visited language, use a coockie
- if a user type an unavailable languagecode in the url, implement a mehtod to
define a default system language.
- then set the language to the current thread

protected void Application_BeginRequest(Object sender, EventArgs e)
    {
        String language = Request.UserLanguages[0];
        if (language.Length > 1) language = language.Substring(0, 2);

        HttpCookie cookie = Request.Cookies["ReBuy"];
        if (cookie != null && cookie["language"] != null)
        {
            language = (String)cookie["language"];
        }
       
        String appName = Request.ApplicationPath.Length > 1 ? Request.ApplicationPath : "";
        String path = this.Request.FilePath.Substring(appName.Length);

        String[] splitPath = path.Split('/');

        if (splitPath.Length > 1)
        {
            if (splitPath[0].Length == 2)
            {
                language = splitPath[0];
            }
            else if (splitPath[1].Length == 2)
            {
                language = splitPath[1];
            }
        }
        language = Bll.Language.Language.GetSystemLanguage(language);

        if (cookie == null)
        {
            cookie = new HttpCookie("myappname");
        }
        if (cookie["language"] == null || cookie["language"].ToString() != language)
        {
            cookie["language"] = language;
            cookie.Expires = DateTime.Now.AddYears(1);
            Response.Cookies.Add(cookie);
        }

        Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture(language);
        Thread.CurrentThread.CurrentUICulture = CultureInfo.CreateSpecificCulture(language);
    }

I hope it helps you.