Andreas, I'd be interested to know, is your Prius the same as the Classic, sold in the U.S., model years 2001, 2, 3? Or is the Original actually a different model than the Classic? And if different, is it significantly different or just minor style changes?
I'm thinking that if it's significantly different we might hope for the next generation as early as the 2007 model year, but that if it's the same or very similar we may have to wait six years.
As to your question, if yours is the same as the Classic, you might look for a copy of the Classic owner's manual. Otherwise, I suggest finding an exchange student from Japan, or the Japanese language department at a local university. Most of my '04 owner's manual is common-sense stuff. The actual how-to stuff is very little, and if you found a Japanese speaker you could probably go through it verbally, taking notes, in half an hour, or an hour at most, and have all the important information.