This is something I’ve given a lot of thought to before, although what I discussed at the time was more of a simple rule builder: Rule Builder
I don’t think you’d want a whole new language, you’d probably want to be able to add JS/Node rules in Monzo (or maybe Go, since that’s what the backend is written in) with a simple API.
Essentially Monzo would build a rule builder UI which would write this script under the hood and there’d be an advanced view for scripting itself.