You'll find that you won't regret having a real pro do it. In reality, it has to be someone really good, it's not enough to know the language.
All the more, if your documentary is going to be based on these interviews, you can't afford to loose the nuances.
I have done some translations of the sound track of documentaries (same issue except on finished products) and I speak Italian perfectly, so you'd be surprised to find out how many shitty jobs are out there. Sometimes, minor misunderstandings leadot translations that are actually saying the contrary of what was said.
Translating from English, the most common problems regard the mis-translation of American idioms or expressions. When you can't translate literally you need to know how to adapt the sentence to the other langauge and or culture, etc.
getting back to your stuff, you need someone who understands the language and the culture, so they can transalte the fine points without loosing any of the texture, if you know what I mean.
When you've done this part, before you start editing, re-read my posts on the editing phase.
Edited Fri 9 May 2008
by Wolfgang Achtner