Senatus PopulusQue Equestria implies a fairly low level of power for the Diarchs… then again, the existence of Chancellor Neighsay could argue for that, although Luna cancelling Nightmare Night would argue against it.

Personally I prefer Luna Rediit[1] if one goes by Google translate, or, as I originally saw it in a fic Luna Revio (or something like that. In any case, “Luna the Returned”… although perhaps that might need to wait a few years/decades for the populous to be better acquainted with her so the phrase would provoke joy at her return, rather than worries or shaming of her for why she left.
[1]Nothing to do with the site.