Edit: Okay, I found it, just needed more focus on the related entry. The package
mathastext called with the
frenchmath option does the job.
The time for my first question to stackexchange could have been delayed...
I've realized that French typography uses a different style for mathematics, namely upright uppercase roman letters and lowercase greek letters (roman lowercase remaining in italic), while default LaTeX is italic. What I want is to use this style with the default math font.
I found far less resources on the Web than I expected, like this one, which introduces math font packages with options for French (or ISO) math style. Nevertheless I'm quite unhappy with the general look of these fonts, not mentioning the fact that unicode-math seems to clash with amssymb.
This related question has been answered with:
Simply put these in your preamble:
It doesn't look good though, so I guess you may also want to change font.
which I guess is interesting for my issue, but not explained a single bit and I don't know how, or if, I can adapt it.