As I have plaid with Artix -dinit for a month, I noticed that if you want to use another language than english in the display manager (I tested LightDM and SDDM), you have to put the wished locale in /etc/environment. It seems that neither SDDM nor LightDM read /etc/locale.conf to choose the displayed locale.
Is it typical for dinit or does the same problem occur under the other init-systems.
In other (systemd) distributions such as fedora, archlinux or opensuse, you just have to configure the /etc/locale.conf file.
A possible solution for changing sddm's language on a non-systemd system:
SDDM : Wrong language at login (https://www.linuxquestions.org/questions/slackware-arm-108/sddm-wrong-language-at-login-4175694295/)