Do you have to speak a language well to translate it?
The short answer is, it doesn't hurt.
Of course it's not an absolute prerequisite. As a counterexample, lots of people put out fine translations of dead languages — languages nobody speaks any more.
But speaking a language well implies the ability to quickly comprehend and manipulate a rich variety of language constructs. Knowing the language at this gut level makes you a much faster reader, and less likely to make wild misinterpretations because you don't know common phrases and idioms.
I'm not talking about the mythical "perfect accent" here. Andrei Codrescu is a perfect example of someone with a notable foreign accent in English, but whose mastery of the spoken and written language surpasses that of almost all native speakers.
Now, I just need to convince the tax office that 飲み会 are a business expense because the conversation improves my translation ability…
[...] and mimic the accent of Japanese. But my accent hasn’t really come into play professionally. Knowledge of spoken Japanese, yes — but not my own pronunciation of [...]