Sometimes people's lawyers will also advise them not to accept being questioned in a foreign language even if they're relatively fluent because small language nuances may make a big difference in legal proceedings. I've given lectures and press interviews in Portuguese before, but if I were giving an oral deposition in a Brazilian court case or being questioned by police, I would ask for an interpreter.

I was once on a jury in California in a case where several witnesses gave testimony through a Spanish interpreter. It seemed clear that some of the witnesses who testified through the interpreter were fluent in English, and so it seemed a little gratuitous to me at the time. Later on I understood that it made sense both for reducing the stress in the situation and for avoiding any minor linguistic misunderstandings in cross-examination that a lawyer might try to make a big deal out of. It's quite possible that the lawyers encouraged their witnesses to use the interpreter.