But that's my point. You'll stop, switch, speak, stop, switch, resume. You're not going to be "I was in 東京 yesterday" as a single continuous sentence. It'll have to be broken up to three separate sentences spoken back to back, even for humans.
I think this is totally wrong. When you have both parties speaking multiple languages this happens all the time. You see this more with English being the loaner more often than it is the borrower, due to the reach that the language has. Listen to an Indian or Filipino speak for a while, it's interspersed with English words ALL the time. It happens less in English as there is not the universal knowledge base of one specific other language, but it does happen sometimes when searching for a certain, je ne sais pas.
Not really, most multilinguals switch between languages so seamlessly that you wouldn't even notice it! It even has given birth to new "languages", take for example Hinglish!!
>"I was in 東京 yesterday"
I think it's the wrong example, because this is actually very common if you're a Chinese speaker.
Actually, people tend to say the name of the cities in their own countries in their native language.
> I went to Nantes [0], to eat some kouign-amann [1].
As a French, both [0] and [1] will be spoken the French way on the fly in the sentence, while the other words are in English. Switching happens without any pause whatsoever (because there is really only one single way to pronounce those names in my mind, no thinking required).
Note that with Speech Recognition, it is fairly common to have models understanding language switches within a sentence like with Parakeet.