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.