Vocal music major here. It is quite likely that by the time they reach university age, students who are majoring in vocal music have had years of experience singing in a variety of languages. These would typically be Latin, Italian, French and German, along with their native language. This might include choral and solo performance experience.

I suspect they have a combination of experience, learned 'coachability', and a greater than average awareness of how to produce the correct sounds.

Anecdote: I'm an English speaker, and traveled to The Netherlands on business a few years ago. It was my impression that just about all the Dutch speak English ( and a few other languages ) and it is very difficult for a non-native speaker to attempt to engage them in a conversation in Dutch. They will usually just flip over into English. After a bit of practicing on the plane, I was able to do a few things like order tickets for a movie, and some snacks, and even order food in a restaurant in Dutch (I cheated a bit here, it was a Mexican place, so all the food was in Spanish :) ). It is possible they were just being polite, but my perception was that I came off like a maybe-fluent non-native speaker. In fact I knew about 20 words, but I had a pretty good accent.