I think it happens quite frequently, because if you're not that sure of yourself, you get reassurance from listening to the translator. You may get it right, but that's a backup for you.
As for being functionally bilingual, I take it more as being conversational. Legal arguments are not conversational. They're quite different, as you know, so the translation acts as a backup. I don't think being functionally bilingual would be a great deal of help. Being completely bilingual is a different thing. Completely bilingual in both languages--well, that's a requirement that certainly would elevate the court if you had competency and bilingual ability both.