After installing CentOS 7 with German locale the keymap still was set to us. To change it to de I had to edit the file /etc/default/grub and replace us with de after vconsole.keymap=:
GRUB_CMDLINE_LINUX="... vconsole.keymap=de ..."
After that execute the following command to update grub and reboot:
grub2-mkconfig -o /boot/grub2/grub.cfg
Of course you have to take these steps as root or with sudo.