How can I get the local group name for guests/administrators?

The names of groups are localised depending on system language.
For 'well known' groups like 'Administrators' and 'Guests' you should retrieve based on the SID. The SID for Guests is:
S-1-5-32-546
There is a list of well known SIDs here:
http://support.microsoft.com/kb/243330


Looking up the account by SID is the best way to go. It's a bit contrived, but the way it works is this:
The Administrator account's SID always starts with S-1-5-21 and ends with -500. Everything else in-between is random (the domain's SID).

The Guest account's SID always starts with S-1-5-21 and ends with -501.

System.Security.Principal.WindowsIdentity currentUser = System.Security.Principal.WindowsIdentity.GetCurrent();

var sid = currentUser.User.ToString().ToUpper();
 if (sid.StartsWith("S-1-5-21") && sid.EndsWith("-501"))
 {
   return "Guest";

 }

WindowsPrincipal wp = 
                new WindowsPrincipal(currentUser);
 
            Console.WriteLine(wp.IsInRole(
                WindowsBuiltInRole.Guest));
 
            Console.WriteLine(wp.IsInRole(
                WindowsBuiltInRole.Administrator));
 
            Console.WriteLine(wp.IsInRole(
                WindowsBuiltInRole.PowerUser));