The DGAC does accept other ICAO IRs, including the FAA IR, and it will endorse such licences for use in French aircraft, provided that the licence holder is not an EU resident.
Meaning that if you're
Resident legally outside the EU, for example a legal Alien in the USA [that cracks me up
] then you can apply for an endorsement even if you're a brit. Its true I'm afraid, I asked nos frères et soeurs français à DGAC if I could use my IR in a G or F reg plane. To the G reg their response was "whatever the UK say you can do" and to the F reg one, only if I was legally resident outside the EU and could prove it.....
Cyer
EA