I added a postgresql service to the list of my services, like this
(services (cons* (postgresql-service #:locale "it_IT.UTF-8")
(gnome-desktop-service)
(xfce-desktop-service)
%desktop-services))
This is an excerpt of the "guix system reconfigure etc/config.scm" log
...
loading system objects' descriptions ... ok
creating collations ... sh: locale: command not found
ok
No usable system locales were found.
Use the option "--debug" to see details.
...
shepherd: Service user-homes could not be started.
...