bug#40273: installer: No way to input Latin characters with non-Latin keyboard layouts

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

After selecting non-Latin keyboard layouts in the graphical installer,
there is no way to input Latin characters again.  This means hostname,
username, password, the home directory path /home/… cannot be
specified in Latin.

There either should be a way to switch the keyboard layout later on.
For example currently there is neither a place in the GUI to switch
the layout nor key combinations or (on a Japanese keyboard) support
for the keyboard’s romaji key to switch layouts.  Of course typing
`loadkeys` is impossible too.

Or the keyboard layout should not be switched to non-Latin.  For
example, when choosing Arabic AZERTY layout, during install the
keyboard should be switched to Latin AZERTY layout and not Arabic.

Note that choosing non-Latin usernames and passwords appears to have
caused problems before, see <https://bugs.gnu.org/37872> (I have not
tested it).

Similar considerations: Maybe the default keyboard layout for virtual
console TTYs and bootloaders should be Latin unless changed
explicitly.

Regards,
Florian