So as I write my grandfather's stories, I'm constantly having to decide between using French vs. English words. I don't like it when books that take place in France (but are written in English) use common French words (like "oui" instead of "yes") to make it seem "très français" -- that practice seems pretentious and unnecessary. Besides, I think the use of French words in an English-language book can be distracting, and tends to pull the reader out of the world of the book. If you notice a word is in French, it means the flow of information, the telepathy (as Stephen King calls it) between the writer and the reader stumbles slightly.
So, my default is English. Besides, that's the language I know well.
There are exceptions (because of course there must be exceptions. Sigh...)
The most important exception: proper nouns. The name of the town is "Beaumont-lès-Valence," NOT "Beautiful Mountain of Valence." Étoile is the name of the town, not "Star." Dr. Jean Planas, is named Jean, not John. His son Michel is named Michel, and not Michael. My name is Cathy Byland Weeks, not Cathy Byland Semaines.
When the word in French is actually better than the English: maternité is "birth center" or "maternity hospital/clinic" in English. I like the word, and that it's a single word, and "birth center" seems too modern a term. So, I used maternité. But now I'm reconsidering this one. Not sure yet. Maternité or maternity clinic -- what do you think?
When the proper noun is a descriptive name of an organization: I haven't decided on Secret Army vs Armée secrète. There's nothing to be gained by using the French, I don't think, except that "Secret Army" seems a little corny.
Codenames: My grandfather's commanding officer was codenamed "Sanglier," which means "boar" or "wild boar." And his son served in the same maquis unit as head of the medical/first aid group. His codename was "Marcassin." Which means ... "young wild boar." So, they were ... Boar and Baby Boar, and that is some wonderful humor. If I use the French words, as I've been doing, because they were in effect *names*, the humor is lost on English speakers. But if I use the English words, it seems ... a little corny. I am leaning in that direction though (damn it. I've got Sanglier all through the text).
Historical acronyms: in France, during WW2, there was the STO (Service du travail obligatoire) or "Compulsory Work Service" where Frenchmen were sent to work camps in Germany because Hitler drafted so many German men into the Wehrmacht, that there weren't enough workers to keep the German economy going (let alone a war economy), so the Nazis demanded France, Poland, and Russia make up the difference and provide them with workers. In the end, I decided refer to it as STO (with a footnote to explain the acronym the first time it occurs), but describe it as compulsory work service, conscription, work camps, etc. I could make up an acronym for English, maybe "CWS," but ... STO was used commonly and CWS has no historical usage at all.