That sounds worse than either option. At that point put it all in one repo with a directory for each language.
And then at some point your Rust people write a Python module in Rust via pyo3, and it has to be integrated into Python build system and Python type checkers, but also needs local rust crates as build dependencies and local python packages as runtime dependencies... hm.
Coupling and Cohesion likely has nothing to do with the language.
This will start to become a problem if the stacks need to communicate with each other using versioned protocols.
Why can't you just use versioning in your external-to-the-monorepo APIs and use HEAD within the monorepo? Nothing about combining some projects into a monorepo forces you into dropping everything else we know about software release cycles.
The point is that it is more work.
More work than what? More work than sharing HEAD in the monorepo, certainly. But it's definitely not more work than versioning across multiple repos because it's literally the same thing. When you're exporting code from a monorepo you follow all the same patterns you would from a small single library repo.
Maybe I miss the point here, but it seems to me that versioning the protocols is the specific solution to maintaining interop between different implementations.