Documentation is currently lacking for internationalisation procedures under TG2 — no information on how the new system works, integration, guides for new users, &c.
you may be searching for that:
http://www.turbogears.org/2.0/docs/main/Internationalization.html
it's still rough around the edges but better than nothing...
https://github.com/TurboGears/tg2docs/issues/11