It's in the 1988 version of the Official Languages Act, section 16, which reads as follows:
16.(1) Every federal court, other than the Supreme Court of Canada, has the duty to ensure that (a) if English is the language chosen by the parties for proceedings conducted before it in any particular case, every judge or other officer who hears those proceedings is able to understand English without the assistance of an interpreter; (b) if French is the language chosen by the parties for proceedings conducted before it...
So it's up to the courts, except for the Supreme Court, in accordance with the version of the act amended in 1988.