It just so happens that I work as a freelance interpreter, and I am accredited to work in the Quebec Court system. Most of what is said in the other posts is pretty much correct.
Educaloi is a non-profiy organization whose mission is to inform Quebecers of their rights and obligations by providing legal information in everyday language. You can consult their website here (click on the “English” tab at the top of the page to read their stuff in English).
The best summary I can find of language rights in the courts is the following, from their site:
"The Charter of the French Language (better known as “Bill 101”) confirms the constitutional right of using either French or English in any pleading or process before the courts. It actually goes further than the Constitution, by guaranteeing the free translation of any judgment from English to French, or vice versa, at the request of one of the parties. In some cases, the judge may have his decision translated even before it is officially released.
Also, as mentioned above, an accused may choose English as the language of trial. People who have been granted an acquittal or pardon in the case of criminal charges may ask that their file be translated to have proof of their good conduct for American Customs.
The Ministère de la justice du Québec will provide an interpreter to an accused who wants to testify in his own language or does not understand the language used by a witness. However, such free services are limited to criminal cases. In civil matters, as in all other provinces, you have to pay for your own interpreter. "
Most Quebec judges, lawyers and prosecutors can speak English (some with a heavy accent). What can and does happen, though, is that one of the witnesses at a trial held in English may speak only French. In this case, my job is to sit next to the accused and whisper to them in English what the witness is saying.
A funny thing that sometimes happens is that the accused is very upset at what he considers untrue testimony by a witness, and he grabs my arm and says: “THat’s not true, object!” as if I were his lawyer, instead of his interpreter.
Another rule is that I am bound to tell the court everything the accused says to me. I cannot edit. So I have to warn them that before the trial that if they whisper to me “the Judge looks like a mean bastard”, I have to get up and repeat that comment in the other language!