Would there be any young French Canadians (maybe from small towns) who went through the Q and stayed completely monolingual? I guess that path to the NHL includes some time in the AHL for most guys, which is where you'd start learning for sure.
Not so much about language, per se, but more about accents. Here is Patrice Bergeron circa 2003:
Here he is today:
For those who aren't Pierre Mcguire, he did grow up playing in the Q. Honestly, I don't remember how his English was in spontaneous interviews in 2003, but I really doubt he ever used a translator. And he's dropped almost all the accent...on the Quebecois accent scale, he's closer to "Well trained French-Canadian broadcaster on US NHL Network" than he is to, say, Celine Dion. You still get an occasional "dis" and "dee" and "dere" instead of "this" and "the" and "there", but, personally, I have to listen hard for it.
He definitely didn't stay monolingual, but my guess (and I could be wrong) is that English was his "school language" as a kid, and that he ramped up his English, either naturally via immersion or with lessons, once he made the NHL. He also had the benefit of time, playing a year in the AHL after making the NHL, thanks to Uncle Gary and the 2004-5 lockout.