Unfortunately, there aren’t that many films in Welsh to begin with, so I doubt Netflix believes it’d be worth their money to acquire licences to show those films on its site. Unfortunately, I suspect buying the DVDs and waiting for S4C to broadcast them are the only ways to legally watch them at home.
I had never seen Solomon and Gaenor either but I’ve just enjoyed sitting here watching it - so sad, but such a good film. Unfortunately the version posted above comes with subtitles that look like they might be Polish, so it gets tricky in the Yiddish parts. I can understand enough German to get quite a lot of it, and some meaning can be guessed from the context, but I still didn’t get everything.