I've added the following code to my master template in order to redirect first time users to the correct language site and it works fine from all computers I've tried, but my customer says they're getting redirected to the wrong site (which is the fallback default site).

<script Language="C#" runat="server">
    protected void Page_Load(object sender, EventArgs e) {
        if(Request.UserLanguages != null && Request.ServerVariables["script_name"] == "/") {
            string language = Request.UserLanguages[0];
            if(!string.IsNullOrEmpty(language)) {
                //Get the culture name - and make sure it's in the RFC 1766 standard <languagecode2>-<country/regioncode2>
                //i.e. en-GB, so it's easier to check for
                if (language.Length < 3)
                    language += "-" + language.ToUpper();
                //Redirect to the correct page based on language, default to english
                switch(language) {
                    case "en-US":
                        Response.Redirect("/en/");
                        break;
                    case "nb-NO":
                        Response.Redirect("/no/");
                        break;
                    case "nn-NO":
                        Response.Redirect("/no/");
                        break;
                    default:
                        Response.Redirect("/en/");
                        break;
                }
            }
        }
    }
</script>

Anyone knows what can cause this? Any help appreciated... ;)