[simonw]

One of the neat new features of Observable Framework is you can drop in a build script to gather the data that's written in any language you like. https://observablehq.com/framework/loaders

So you absolutely can do the data processing step in R or Python and have that output JSON or CSV which is then visualized at the end using JavaScript.

Not a small feature, but I bet it would be possible to use WebAssembly to add support for Markdown blocks that get executed in other languages as well, using Pyodide for Python for example.