The paradigms from different programming languages aren't always compatible. The differences in those paradigms are the strengths and weaknesses that make languages better or worse for various applications. It's hard for me to imagine low-level plumbing that would give you the languages' strengths when mixing languages -- some mixes just don't make sense.

As an example... why would I want to call python from C? If I'm writing in C, its because I want high performance with low-level control of resources. To call a python function, I suddenly need to spin up the python virtual machine including its baggage like its garbage collector -- there goes my performance and low-level control, I'm suddenly running a massive stack that I didn't write!

Alternately, you could compile the python code into a bytecode (different than PVM bytecode) that C can call performantly... but to do that you lose the benefits of python. Suddenly your python code needs to be compiled after every change (slowing down development, one of Python's greatest benefits) and you're required to declare and enforce strict types (no more easy duck typing!) so that C can count on getting back data in the expected format.

In contrast, calling C from Python does make sense and can already be done.

And writing plumbing to handle generic interoperability between 2 languages would be a lot of work. Writing parts of a project or system in different languages and then having them communicate through a standard interface like an API is much less work.