I know that there are ways to assess the linguistic competency of people. I'm not a specialist in these evaluation methods. I think this is a matter for translation or education specialists.
I believe I understood that in the context of the process you mention, questions are put to the candidates, and that the government, through the Office of the Commissioner for Federal Judicial Affairs, reserves the right to have them undergo tests. I don't know if that was ever done, but I expect that it was not necessary for certain candidates, given their experience. In the case of someone who has diplomas from both an anglophone university and a francophone one, it may have been deemed unnecessary to go further, given the circumstances. I don't know what policy the Office of the Commissioner for Federal Judicial Affairs adopted in such cases. It could be useful to administer various types of tests.