Credits: @Torxed archlinux/archinstall#107
The keymap set in the in the archinstall script does not work in your window manager.
Solution (edit the keymap "de" with the one you want):
$ localectl set-keymap "" && localectl set-keymap de
$ localectl set-keymap de && localectl set-x11-keymap de
After that, reboot your machine.