It wouldn't be possible to "Anglicise" pure foreign words like tobacco, anchovy, or alligator, bungalow or pyjama, kangaroo or boomerang which leapt into English precisely because we had no relevant native term.
Bon vivant, mot juste, au fait, pas de deux, la vie might seem pretentious because they do have equivalents, but the French phrases tend to be shorter, sharper and better suited to the purpose. In French, bon vivant might be a tiny twist away from "good living/liver" but a real English equivalent would clutter itself up with "a person who enjoys (living) the good life” or "a connoisseur of food and drink…" quite a jumble even without "… and of the other things that go along with a life of leisure and luxury…" (dictionary.com/browse/bon-vivant)
Others simply take a long time to bed themselves in. Even 60 years ago, we Brits chose on the spot whether to spell hôtel with a French accent and silence the "H"; whether a theatrical part was a rôle, and either might still have been written in italics. Nowadays, who minds?
The meaning might be obvious but could "long chair" be as warm or cosy as chaise longue?
In general, it's a question of usage. If it's taken up by fewer people, the loan-term will keep its original associations. The more it's taken up, the more it makes itself at home, as when the phrase "de bon aire" collapsing into "debonair".
I think "we all know" that today, "debonair" is having a gay or light-hearted atmosphere about one… "gay" in its traditional sense
I think "we all know" that French "de bon aire" translates literally as "of/having a good air/atmosphere"…
Please note, though, most of Google's resources insist the original French "… aire" referred to family, breeding or manners, leaving only a faint trace of itself.