Multiple languages in single notebook

[Peter Keše]

There are several communities building jupyter compatible servers supporting multiple languages within single ipynb file:

https://vatlab.github.io/blog/post/sos-notebook/
https://polynote.org/latest/docs/mixing-programming-languages/
https://github.com/dotnet/interactive/blob/main/docs/polyglot.md
Julia, R, Javascript, others.

All of them are inventing their own hacks and workarounds to circumvent the fact that nbformat does not provide support for multiple languages or multiple kernels.
Which in turn makes these custom notebook formats incompatible with the rest of the ecosystem, e.g. jupyter nbconvert --to html mynotebook.ipynb doesn't work on most of these files.

Please consider fixing this situation and adding proper support for multilingual notebooks.
At the minimum extend the json schema for .ipynb files.