Last modified: 2012-08-23 22:13:38 UTC
Siebrand already created the extension. ISO3166 can be found at https://en.wikipedia.org/wiki/ISO_3166-2 It should probably just be a simple php array of messages with the form: <prefix>-<iso code>. Just a few country (us, nl, de) and some subcountry codes should be added (us-ny, us-dc, nl-nh) to start with.
FWIW, Siebrand 'created' the extension in that he created a git repo for it - but there are currently no files in it.
We wound up implementing this solution a little differently. Because we're hosting the API independently of MediaWiki, it did not really make sense for us to implement this as a MediaWiki extension: https://github.com/awjrichards/WLM-ISO-3166-2