How can I change the locale in GDM?

Hi there,

I using GNOME 3.22.2 and GDM 3.22.1 on Apricity OS, which is basically Arch Linux. I have posted this question in the Arch forums but I got no reply so far, so I decided to try here.

I want to change the locale of GDM.

I edited the /var/lib/AccountsService/users/gdm and I rebooted, but nothing changes. That was suggested at the Arch wiki: https://wiki.archlinux.org/index.php/GDM#Change_the_language