The trouble is contributors generally don’t know all three languages. They’d need to take PRs on their PR branch for the others. Making feature branches on allwpilib is a non-starter because we can’t give people push permissions like that and trust them to not break the repo history. People can be very bad at Git.
If you make a PR intended to go on a feature branch, a maintainer would need to make the branch first, which makes initial contributions harder.
I guess the Linux kernel solves all this with a staging area.