Subdomains country specific 'subdirectories' help!

I want to set up a multisite with subdomains e.g. site1.example.com, site2.example.com etc but have each be able to be country specific i.e. site1.example.com/us/, site1.example.com/nz/, site2.example.com/us/ etc.

http://marketplace.xbox.com/en-US/ is a great example of this.

Basically I need to seperate my main site into smaller subject based sites (the subdomains) but the content that is served is on a country basis so that anyone in the US who goes to any of the subdomain sites will end up with the US specific content as a ‘subdirectory’.

I don’t care if the subdirectory URL is just there for looks, just so long as it serves the correct content.

However I’m not sure how to set this up. Any help is very much appreciated!! Thanks in advance 🙂