I think it would be amusing, if, with as much time as the kids spend with Maria, they pick up on her expressions and the words she uses. For example if they're misbehaving and she utters something under her breath, the kids associate the words with the behavior. Then.... down the line they repeat it to Daddy and thing its grandly funny Daddy doesn't know what they said.
This is how kids operate. I know of a child that was quite small that emigrated to the US and knew Arabic as his first language. He actually thought that "s**t" was a commonly used word. When you think about it, in the English language, it actually is by a lot of people. He mimicked what he heard
Teaching the language in a school environment teaches the word association, what it means and how it is used grammatically.
Here in the US, other languages aren't a focal point of education all that much. Most Americans have problems with the English language even after completing high school levels.
Me? I had German in elementary school for a bit and although I retain some of it, I would be hard pressed to say I know the German language at all. I had several years of French in high school and pursued it a bit further in college, it serves me well sometimes but I'm far, far away from being fluent. I'm still interested in etymology but that's just me. With journalism training and a love for words, I love finding out where they started from. Its a side interest.
My kids grew up learning Spanish and French a bit only by watching Sesame Street (it depended on the station carrying it if it was French or Spanish). I grew up knowing to say "x, y, zud" crossing the border to Canada. The reason escapes me now as it was so long ago.
We pick things up as we go and that's true for adults. I will never have to be fluent in another language because of my lifestyle. My interest as a young adult was in words. English grammar, English composition, English literature and Journalism training. It was specific to my interest. It's what drew me.
Perhaps William isn't fluent in another language. Perhaps it didn't interest him all that much how to speak a spoken word. He did, however, pursue things that interested him personally. I can't fault him for that.