I have an idea. What about to create a project on Launchpad and import to it transcript of screencast every time a new screencast is released (or before)? You could use Rosetta to get translations! Every screencast could be displayed as a separate translation template. It would be very easy way to create subtitles (timing would be done only once, by you) and I think a lot of interested translators would participate. I would.

I don't know if translations can be incorporated directly into ogg file, but you can always publish them as external files on these pages, even after screencast release.

What do you say?