Preferably one who actually know Star Trek, otherwise the translated versions will end up with new (and possibly valid) new word constructs.
In addition someone who knows Star Trek will know which words not to translate, example:
TNG "Redemption", they translated disruptor as "Unterbrecher"...well this is the correct term but it's also incredibly silly because it sounds like a piece of the ship's engine is broken, noone would ever guess that was a weapon. So in most cases it's not translated and left as "disruptor" (thankfully!).
I shat bricks when iread that in a TNG novel. if thoughts could strangle people....