As far as I know, nobody translated the API reference so far. It’s automatically generated from comments in source code. Currently there is a script for keeping track of changes in The Definitive Guide to Yii (and also for Blog Tutorial).
There is a Russian translation of API but personally I think that it doesn’t worth it since that’s too much effort. As for the guide and blog tutorial, these are displayed at the website and it doesn’t take too much time to keep these up to date after initial translation is done. As tri already mentioned, project is hosted at google code. If you want to participate, let me know and I’ll add you to project members.
I decided not to translate the api documents because it can be very tedious and boring, but I believe that it will be very meaningful if I join the project "yiidoc".