We will go to the rendez-vous as madame demands it.
What do you call the act of using a word from another language for stylistic effect like above as seen with the word bolded in black? I am pretty confident there's such a word, but I don't have the slightest idea what it might be.
Collectively words "borrowed" from other languages are called "loan words". Rendezvous is one such, so much so that it's an actual English word:
rendezvous (n): A meeting at an agreed time and place.
There are thousands of other such words: barbecue, champagne, faux pas, kitsch, modus operandi, schadenfreude and many others.
However if you mean dropping actual foreign language words or phrases into a sentence, either to sound cosmopolitan, or because the words just don't translate well, then there really is no single term to describe this. For example:
I was so angry I couldn't speak, and it wasn't until yesterday that I had l'esprit d'escalier, as the French say, and thought of what would have been a perfect response.
There are various terms for what other people might think of it, such as "elegant" or "pretentious" but the practice itself is not called anything.
A neutral description for this is "code-switching"--where a person alternates between languages or dialects in a single conversation.
It can be done for stylistic purposes, as a marker of class or community, for technical reasons, to change the degree of formality, etc.
If you're looking for a word that implies that this is a little pretentious or affected, however, I'd say that "code-switching" wouldn't meet the criteria. The term by itself doesn't imply anything about the speaker's motivations for switching.
I'd go with preciosity:
preciosity n
1. Overrefinement, as in language, taste, or style
2. An instance of overrefinement
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