Looking at the code it appears to be somewhat easy, you add your language to language.rs and in the engine folder you add yourlang.rs where you provide an implementation of the LanguageEngine trait for the YourLangEngine struct.

It would be less tedious if some code was factored out into an Helper struct but it doesn't look like it's hard.

It's simple. It was made by someone who's just starting out with Rust.