BCF, I'm just back from a long day at work, but this was my experience of applying for French nationality by decree (more complex than by marriage to a French national; I should have done it before I got divorced...)
I had to provide lots of paperwork and details, including every job I'd had in France since arriving and every one of the 15 addresses I'd lived at, with dates of arrival and departure, (I presume no one checked that bit, as some of my replies were a bit creative...), my parents' birth and marriage certificates, my birth and marriage certificates, evidence of divorce, my kids' birth certificates, proof that I'd paid all due taxes, etc.
Fortunately, I have a friend who is a certified translator and whom I often help out with translations, so he translated any English language documents into French for free.
After 5 months, I was called to the gendarmerie for an interview. I was asked about why I wanted to become French, how involved I was in the local community, etc.
I presume he was assessing my ability to speak French at the same time.
I didn't have to sit a language test as I have a PhD from a French university, which was deemed evidence of my language skills.
I did, though, have to go to the Préfecture, a 100km round trip, and take a test with questions about French literature/culture/history, whether same sex marriage is allowed, whether any religions are banned in France, and other quite straightforward points. I'd revised extensively and was ready to reel off dates for the Revocation of the Edict of Nantes, the various stages of the Revolution, the legalisation of abortion, the introduction of free education for all, etc., as well as the names of local/national politicians and the details of which layers of government are responsible for what, because those things were in the sample tests I'd seen online, but none of that was needed on the day.
Then I waited for 13 months.