Have you ever gotten a product and you could obviously tell that the person who wrote the manual didn't speak English natively?
From my little knowledge of languages it seems that it works better for people to translate into their native language.
I think the same principles applies to developers translating from code to documentation. I would prefer to explain how a product works to a user and have the user write the documentation because they will cover things that seem obvious to me but not the average user.
Someone not intimately familiar with the product will probably write much better documentation.