Headcannon time: Gwen's mother insisted Gwen learn other languages ("Can't have you looking like an idiot in front of your father's clients, Gwendolyn) and one day Boomer walks in on Gwennie making breakfast and singing a song in French. To her surprise he starts singing along, and explains that his mother had family who were French and he picked it up from her when he was younger.
Actually *clears throat* my Owen is half Polynesian, his mother is (Māori to be exact.) He, unfortunately, got his looks from his father, aside from his eyes. It’s why he’s over half a foot taller and much much heavier than his father. (Owen’s like 6′3″ and weighs 240+ lbs) I believe that his mother did teach him some Māori, enough to be able to converse relatively in it. I think he’ll talk to himself in it, sometimes at night (I don’t know the language and don’t trust myself enough to try, unfortunately) just so he doesn’t lose what his mum gave to him, even though it isn’t much.
However, I can see Gwen being at least conversant in Spanish and French (maybe even German), probably not fluent in any because she doesn’t use them often, if at all, because her mother wants her to be, as you said. I can see Owen knowing a little bit of French, or Greek, or really several different languages because of his time in Sydney and him running around continuously. So I can see him knowing songs (I also believe Owen really does like knowledge, and knows the Classics and Shakespeare, he just tends to keep that under wraps.)