Updated: 10/12/2012 6:54pm
I've been working on the LanguagePrefix module, based on this article, and it is now on GitHub. It supports SilverStripe v. 3.x, is fully documented (doc/en/index.md) and has undergone unittesting (read the README in the tests directory for more info).
I consider the module to be in its beta stages - although it is fully functional. Any comments are welcome, please leave them on GitHub as issues - the comments on this site are still disabled due to serious spamming.
SilverStripe CMS and Framework version 3.0+