Fornjot is architected to be language-agnostic. Rust is just the easiest language to support, given that the rest of the project is written in it too.

I (author of Fornjot) don't have any near-term plans to add support for other languages, but I hope that people will start experimenting (and someone already has: https://github.com/WeirdConstructor/Fornjot/tree/wlambda_int...).