It looks like French is one of the harder cases around for Title Case rules, and (perhaps consequently) also in demand. There also doesn't seem to be a Rust implementation yet. This one in JavaScript is evidently the best but even that highlights the complexity and disagreement over style guides.

If this library is going to be usable in places like SILE or Typst, it ought to have more than just English and Turkish. Especially since SILE has French contributors it seems like a likely candidate to try to implement here.