I had the same problem, which was solved by installing glibc-utf8-locales in root's profile and adding the export line to root's ~/.profile, which fits Ricardo's explanations. 

Le jeu. 17 janv. 2019 à 21:56, Quiliro Ordonez <quiliro@riseup.net> a écrit :
Thank you for this straightforward answer Ricardo. I post some insights
below.

El 2019-01-17 09:24, Ricardo Wurmus escribió:
> This is in your environment.  But what about the daemon’s environment?
> It is not started in your environment where GUIX_LOCPATH is set.  How do
> you start the daemon?  If it’s via systemd you may want to look at the
> service file to see where GUIX_LOCPATH points.

In fact:
echo $GUIX_LOCPATH
gives no output as root (which runs the Guix daemon, presumably).

Where is the service file supposed to be and how to include GUIX_LOCPATH
in it?

I also ran the script to install Guix. Perhaps this script should do all
the setup.