It’s different depending on whether it’s in the federal courts or the provincial courts.
Most matters are in the provincial courts, where language use varies. But, the starting point is that most criminal trials don’t result in written reasons at all. If it’s a jury trial, then it’s just a verdict: guilty or not guilty. Judge alone trials are often judgments from the bench, no written reasons. Same for a lot of civil matters, like family law and child protection cases. It’s only if there is a tricky point of law that you’ll get written reasons.
As you go up to the provincial appellate courts, it’s much more on legal points that are in issue, so much more likely that you’ll get written reasons. Whether the reasons are bilingual or not will depend on the province. The courts in New Brunswick, Quebec and Manitoba all have a constitutional obligation to use both languages, but the judges can choose which language to use in their judgments. In Ontario and Saskatchewan, there is a similar statutory regime, where the judges can use either language.
In my personal experience, if it’s a criminal case, the court will give judgment in the language of the accused person. (Speaking of appeals; the accused has a right to trial in their native language, English or French, across the country, and the trial judgment will be in that language). The criminal cases I’ve argued in French, were all decided in French, but some of them were given bilingually as well. If it’s a civil case where only one language is used by the parties, the court gives judgment in that language, normally English in my province. If both languages were used, the judgment will likely be a bilingual one. Don’t know how it works in other provinces were either language can be used, but I imagine it’s similar. Since Quebec and New Brunswick have larger minority language groups, I imagine there will be more judgments in bilingual format. Not sure if NB courts must give bilingual judgments.
It’s different in the federal courts. Under the Constitution, either language can be used in all federal courts. Parliament has added to that, by providing that the judgments have to be rendered in both languages. That means trial decisions of the Federal Court, and appellate decisions of the Federal Court of Appeal and the Supreme Court of Canada, will be published in both languages in the official reporters. (Also the Tax Court and the Court Martial Appeals Court.)
But here’s the thing: most lawyers who go to court normally only have to be familiar with the law in their own province, and any pronouncements by the Supreme Court on the relevant legal issue, so the language issue normally doesn’t arise. The one area where cases from other provinces may have an impact is the criminal law, which is federal and therefore common across the country. For that, there are annotated Criminal Codes, produced by private legal textbook companies, with short summaries of cases from across the country, in both languages:
https://store.thomsonreuters.ca/en-ca/pdp/martins-annual-criminal-code-2021-edition/30913650
If a criminal lawyer needs to get the actual case cited in the annotations, they will be available free on www.CanLII.org. They’re normally in the original language, so if the leading case on a particular section is from the Quebec Court of Appeal and is in French, anglophone counsel and the trial court in another province will have to make do somehow. But that doesn’t happen much, in my experience.
Not sure how it works the other way, for Quebec lawyers and the leading case is in English from another province; I imagine that may happen more often, simply because there are nine appellate courts which give the bulk of their decisions in English. However, my personal experience with counsel from Quebec is that they tend to have a high bilingualism rate, in part from studying law. (That may be sampling bias on my part.)