I had similar issue:
$ gnome-terminal
(process:2410): Gtk-WARNING **: Locale not supported by C library.
Using the fallback 'C' locale.Error constructing proxy for
org.gnome.Terminal:/org/gnome/Terminal/Factory0: Error calling
StartServiceByName for org.gnome.Terminal:
GDBus.Error:org.freedesktop.DBus.Error.Spawn.ChildExited: Process
org.gnome.Terminal exited with status 8
I was unable to fix it with `dpkg-reconfigure locales` or `localectl`.
I noticed that LC_ALL was unset so I added it to /etc/default/locale.
$ cat /etc/default/locale
LANG=en_US.UTF-8
LC_ALL=en_US.UTF-8
Gnome terminal works now (after restart).
