[...] because only Workstation uses gnome-initial-setup.
Just two random (and stupid?) ideas:
1- Replace Anaconda in Fedora Workstation with a screen that asks your
language and another that asks if you want to "Erase everything and
install Fedora" or "Install Fedora alongside existing OS". All the rest
handled by GNOME Initial Setup.
or
2- Split anaconda into "Anaconda Installer" (language + partitioning)
and "Anaconda Initial Setup" (everything else) and apply this to all
variations/spins (except "Anaconda Initial Setup" to Fedora
Workstation).
:)