I just thought it's pointless obfuscation of text, I'm just learning it's apparently a commonly used phrase
https://news.ycombinator.com/item?id=41086473
Which is fine! Very common on Hacker News.
My suggestion is that you approach this kind of question the way you did in that post, not the way you did in this thread. "I'm not a native speaker, why did you say raison d'être there?" would get your question answered without the downvotes.
English is a magpie language, and there are dozens to hundreds of undigested phrases borrowed from other languages in it, with more arriving all the time.
You are right, my fault for that. English is a dumpster fire sometimes, could have accounted for that
You can't just toss any bit of any other language into English and expect it to fly. "I like Sally, she's got that joie de vivre", that's fine, "I like Sally's new hairstyle, très chic", also works. "Have you met Sally's new chienne? He's un vrai chéri!" is not English, this is code-switching and would only happen between two bilingual speakers.
Are you saying English speakers commonly know those first two phrases?
https://www.merriam-webster.com/dictionary/joie%20de%20vivre
https://en.wiktionary.org/wiki/très_chic
English is a very large language in terms of vocabulary, but I would expect educated people to know joie de vivre and almost anyone to know très chic, although not necessarily how to spell it (and the accent grave is basically an affectation: "proper" English spelling does include various diacritics, but informally it's very rare to actually include them).
Certainly up north I could see those being known, as it's more likely you had French in highschool.
In the south, there are many common Spanish phrases known. I know some French phrases but I have never heard anyone say the ones you mentioned, nor did I know them. I would consider myself "educated".
That being said, I've both heard and used those phrases, and not in an "educated" context. Sometimes they say what you want to say better than another phrase that uses entirely English words.
I'm aware that I said that said I would expect educated people to know joie de vivre and, in fact, I do. I find it somewhat surprising that you claim to have never heard it before, but hey, I believe you. It doesn't change my expectation, though, which is formed by many years of saying it and having the people around me understand what I said. In many US states, including Southern ones, and abroad as well.
But no, they aren't "up north" regionalisms at all, and their existence in the language has nothing to do with American elementary schools having French classes or not. They're known throughout the Commonwealth, for instance. These are just undigested bits of French which live in the language, like a priori is to Latin.