I've got the Pro Micro board from Sparkfun. Here in Belgium we use an azerty keyboard. I've found the modified keymapping for french keyboards. However I don't now how to use the ALT-key modifier in the Keyboard.cpp keymapping file.
File present in: C:\Program Files (x86)\Arduino\libraries\Keyboard\src\Keyboard.cpp
I tried to implement it myself using the CTRL-key modifier analogies.