It's written in Perl - I think Perl is a niche language these days, right? I wrote a version of part of it in Racket, too, which definitely qualifies. I keep thinking I might move development to Zig.

The language is Decl. I think at this point it's more a notation than a language; it relies on a host language for execution. My understanding of what I set out to do has evolved over time, and the past couple of years I've thought of it more as a DSL along the lines of SQL (not in that it's used for database access, for which we already have SQL; I just mean that it's used by embedding it in something else that defines actions).