It's called "code-switching" or "code-mixing", and bilinguals do it all the time. When an immigrant kid does it, you don't call a doctor, you call it adorable.

By the way, it's not switching topic. You just pick the concept closest to what you mean from your combined vocabulary. If you're not paying close attention, you might switch language though (until the next concept you need is from the other language again, at which point you switch back)

And you're aware the paper "Attention is all you need" came out of machine translation research at Google, right? You hold an internal semantic representation and map in and out from arbitrary natural languages. I think the (bi-, tri-, multi-)lingual approach is the only proper way to translate, and this is a hill I will fight on!

Google may have gotten more than they bargained for on that particular translation experiment; though they failed to capitalize on it initially, with OpenAI running with the ball.