I see lots of unanswered questions about layouts and foreign languages for mac os:
And close to zero support nor answers, nor anything in fact, so here's my take on it since I spent quite some time getting it worked !
There is this thread to bypass the keyboard assistant, well this is half the solution in fact.
You need to get the PID of the right layout AND have the correct json.
So for instance, to have working payloads on a french mac, you'll need that fr-mc.json AND use
ATTACKMODE HID VID_0X05AC PID_0X0250