Yes, I only speak one language fluently but I'll still arbitrarily code switch sometimes.

Human language is actually just one vast system, it isn't best understood as a bunch of nicely compartmentalized independent languages. So in a way it would actually be more wrong for an AI model to insist that "Beacoup money" isn't a reasonable thing for the user to want to write, just because those two vocabulary words don't co-exist in a single defined language. The user may or may not know that, but they certainly don't care.