Wrong system language is shown/used

Steps to reproduce

Fresh Archlinux install, installed gnome and gnome-extra packages, enabled gdm.

gnome-shell: 3.26.2 (package 3.26.2+14+g64c857e3f-1).

/etc/locale.conf:

LANG=en_GB.UTF-8
LC_ALL=en_GB.UTF-8
LANGUAGE=en:fr:ja

(man says LC_ALL may not be configured in this file, but that's not the important bit here, or not entirely)

User keymap set to (french) Canadian Multilingual.

What happens

$ echo $LANG
en_US.UTF-8
$ echo $LC_ALL
en_GB.UTF-8
$ echo $LANGUAGE
en:fr:ja

GNOME talks to me in French. In Region & Language in the control center, gnome shows Langue Anglais (États-Unis), Formats États-Unis.

What I expect

I want GNOME to talk to me in "en", I don't care which.

I don't know where the shell picked French in the first place (keymap?), nor US ($LANGUAGE? default language?), but that's en so fine by me. I did not set any language via the control center, yet, waiting to file this issue first.