There's no definition. At least, I haven't seen one so far. I have never seen a definition for what constitutes a bilingual judge or the level of proficiency required to indicate that a judge is bilingual.
As I said, no one is against virtue. At the same time, though, if a legislative requirement is introduced, some anglophone provinces might not have enough bilingual judges. If it becomes necessary to identify a second category—one that designates an individual who is functionally bilingual and therefore not fluently bilingual—are we to understand that some provinces will never have enough bilingual judges for appointment to the Supreme Court? That is what I wonder.