My spouse and I lived in a bunch of countries over the years. We speak Quebec French, English, and Spanish, as well as a smattering of Chinese, Bulgarian, Korean, and a few odds and ends here and there.
We basically speak whatever we think people around us won't understand. Very colloquial Quebec French in non-French-speaking countries, Chinese around white people, Bulgarian around non-white people, or even a cryptic mix of everything when we're not completely sure.
We figure anyone who understands is probably someone we want to know... Hasn't happened very often, but it does happen. So far we weren't saying anything overly embarrassing when we got caught, but we sure as hell have no filter between us because of this!
I wouldn't say we speak in people's faces, but we make comments to each other about random stuff. I would never say something rude about somebody in their faces, but my spouse might go, "Can we go back to the hotel, I really need to take a shit" or something silly and unfiltered like that.
It's rude for spouses to have a private conversation? Would whispering be better? Would it be better if they hid in a cupboard where no one could see them?
Sure, if people suddenly switched languages and then laughed in my face, I would feel bad.
But if it's like the other comments in this post, and it's a couple having a quick word about a private matter, I wouldn't mind. It's not like I should be a part of that conversation
This varies depending on the cultural norms of the country.
Japanese: I have been in high level meetings with a Japanese company. As soon as I walked into the room they all switched to English. Some of their English was weak but they still made the effort. When I commented on how much I appreciated it, surprised Pikachu faces all around. They responded course they would swap, to do otherwise is rude.
In France I have had business meetings with with 8 people around a table all of who all spoke english. 4 of them were native French speakers, 1 polish, 2 Arabic, and me the sole native English speaker. The native French speakers spoke French the entire time. They would swap to English to interrupt the English language conversation then swap back to French amongst themselve. If two or more native French speakers are together, they speak French and don't give a fuck if they include you or not. They then act all surprised that you didn't follow their in French conversation.
It is rude to intentionally speak in a language that the people around you don't understand (especially if you're doing so specifically because they don't understand it), yes.
That's probably a cultural thing, isn't it? In diverse areas, people don't expect to understand what they hear others say, so there's no "Speak ___; we're in ___" culture.
I'd say it's more of a context thing. If you're hanging out in a group of people chatting together and you code switch to speak to someone so nobody else can understand, that's rude. If you're just speaking to someone in another language on your own, nobody cares (except xenophobic bigots).
I wouldn't even include all of America in that either. It's really just a rural America thing. Sure 99.9% of people will speak/understand English, but in my anecdotal experience, it's pretty common near big cities for people to be bilingual and grow up speaking a different language with their families.
I love those videos where people are caught trying to have a private conversation by someone who speaks an unexpected language! Also it's shocking to me how many people loudly speak common dialects of Chinese and don't expect anyone to follow... literally over a billion humans can understand Mandarin, someone is listening.
Haha, I've caught plenty of Chinese speakers having what they presume are private conversations in my presence, and sometimes even about me. People just automatically assume non-Asians can't speak Chinese, even when these non-Asians live in China.
This happens to me sometimes as I lived in enough places that I understand a lot of common stuff in various European languages even though I don't speak them beyond ordering a beer or whatever.