The code you want to use has to compile to Webassembly.

This is in fact possible for C# and Python, but especially in the browser you would want to have a small binary size, and avoid bringing along the whole runtime of these managed languages if all you want is a small functionality subset.

A very viable solution would be creating a decimal package with either a native Rust crate (I think there are a few decimal implementations) or with one of the goto C libraries.

That would be much smaller and faster as well.