gnome-initial-setup should support Ubuntu language packs

[Jeremy Bícha]

The Welcome page (first page) of gnome-initial-setup allows individuals to choose what language they would like to use. This wasn't designed to work with Ubuntu's language pack system.

There are a few patches in gnome-control-center that need to be ported to gnome-initial-setup.

[Jeremy Bícha (jbicha)]

A few comments for reference.

1. This is blocked by LP: #1631750

2. It would duplicate the language selector in the installer

3. For the existing-user mode in gnome-initial-setup, the user would need to log out and log back in for the language change to take effect.

Because of #2 and #3 Fedora 28 has also disabled their language selector for existing-user mode. They did it with their vendor.conf and reduce-initial-setup-redundancy.patch. The language selector is still available there for new-user-mode (what OEMs can use. We could also use it if we tell ubiquity not to set up the initial user.)

https://src.fedoraproject.org/rpms/gnome-initial-setup/tree/f28

existing-user mode runs as an app in the logged in session.
new-user mode is a special mode in GDM so the app runs full screen and the user logs in afterwards. It also includes a page to set the username and password and an optional page to set the timezone too.