I haven't made a programming language (and never will), but I did build a BNF-inspired metalanguage for describing text and binary formats to scratch the itch of trying to describe a binary data format I was developing:

The metalanguage: https://dogma-lang.org/

It's even got a syntax highlighter: https://marketplace.visualstudio.com/items?itemName=ksteneru...

The binary format I wanted to describe: https://github.com/kstenerud/concise-encoding/blob/master/cb...