I have been building a new not-exactly-a-programming-language using Racket and it has been an absolute pleasure, and the community has been extremely friendly and helpful.

One existing stumbling block that I encountered on the way is that I could not find documentation for the full process for the setup for creating a new packaged language (I have been known to fail hard at finding the right documents so this may just be my problem). https://beautifulracket.com/ has excellent coverage of the general process, but the differences between #lang br and #lang racket/base are just large enough that I couldn't fill in the gaps without going and looking through multiple repos to see how others had implemented their repo layout and then could intuit which setup commands I needed to run.

If I find time I intend to write up my findings and submit them to the documentation, the short version is here in case someone finds it useful.

  repo-base
  ├── .git
  ├── my-lang       ; run `raco pkg install` here
  │   └── info.rkt  ; meta package depends on my-lang-lib
  └── my-lang-lib   ; run `raco pkg install` here
      ├── info.rkt  ; include actual dependencies
      └── my-lang   ; implementation goes here
          └── main.rkt  ; in theory you can put your whole implementation here

Once that set up is complete you should be able to use `#lang my-lang`.