As a sometimes motorcycle rider as well, it has been my experience that these are still understood fairly well, at least where I've lived. Left arm is needed because a motorcyclist can't release the throttle to signal with right arm.

I was in my car coming up behind a cyclist who signaled a right turn Mon, so I stayed behind him as I was turning right too. Then he went straight and ran the red! :-(
So I was left somewhat confused as to what he was trying to say with his arm.