Unfortunately Graf is right, there. FreeDoom changes too often to do proper in-engine translation support for it. If you look at the project from even just 5 years ago versus now, they look nothing alike at all. And the changes from 5 years before that are just as big.
So doing all that work now, for a project that ultimately targets simpler source ports (i.e. Chocolate Doom, prBoom, etc) where they cannot handle all of GZDoom's features simply makes no sense, unless the FreeDoom team does this work on their own accord. Things will change, and then GZDoom will be caught left behind with the old work, causing a patchwork mismatch as things are updated.
So while FreeDoom works perfectly fine in GZDoom while in English, having a localized version of it is a lost cause right now.