Last I used WSUS (years ago; beta/v1.0), I didn't think you had to be a member. Can you add it to the workstations via registry insted of a GPO?
It was a pain back in the day to manage and use. Each update had to be approved, and there wasn't a way to filter language packs (imagine each update duplicated by the number of available languages).
Haven't used it since. I've instead gone towards unattended installs, but that doesn't fix your core issue.