The TODO is regarding documentation iirc.

All the languages supported are in the init.example.el file seen here: https://github.com/hlissner/doom-emacs/blob/develop/init.exa...

Uncomment them, run doom refresh. Check their module under https://github.com/hlissner/doom-emacs/tree/develop/modules/... for any external requirements that they need or flags that they support.

The comments are hilarious:

;;hy ; readability of scheme w/ speed of python

;;rust ; Fe2O3.unwrap().unwrap().unwrap().unwrap()

;;javascript ; all(hope(abandon(ye(who(enter(here))))))

From my experience, most of the packages listed there with TODOs have basic support with some extras, but are lacking documentation and testing. For example rust, supports syntax, formatting, lsp or racer, cargo and other goodies.