An interpreter does not only need linguistic ability and knowledge, but also special training in interpreting itself. It is not that anyone who is bilingual can interpret.
Therefore, to say "To become a medical interpreter, someone must be fluent in the primary language of a nation, along with a second language" is incorrect. Being bilingual only gives you the ticket to get on the train of having a possibility of becoming an interpreter.
Unfortunately, this misunderstanding has already placed so much wrong doing in the interpreting process all over the world, especially public service interpreting.