If I had to pick an actual language for configuration I would probably pick Javascript, mostly because I like JSON's syntax.

My instinct would be to say that if you need non-trivial logic, you may have an issue elsewhere with separation of concerns, and should move that code somewhere else.

But I will concede there may be a cases I'm just not aware of where that's unavoidable. Almost everything I do involves json or ini files anyway so I haven't really worked on anything incredibly complex.