Are we ready to break backwards compatibility by making those macros language- context-dependent?
I don't think it would break backwards compatibility very much, especially if the default is anyway C related. The problem is how many changes this would involve throughout Autoconf. It's probably all but trivial, though it can be done.
The answer thus is "show us the patches", even very preliminary ones showing the basic idea. This is how all the changes in 2.64 were done, in practice.
Paolo
