Ok, I have found the answer to my question, I believe that it can be applied to other languages than Portuguese.
The LightDM keyboard layout to Portuguese is done with this command:
localectl set-x11-keymap pt
I did this as
root, and I think it is the right way, but you can do it has a normal user too, I believe.
I've found this on Fedora forum.
set-x11-keymap to work, first, we need to install package
libxkbcommon, without that will give you an error message.
I've found it on ArchLinux forum.
I asked this question also in Arch Linux forum and this answer, solves the problem too, adding to the file /etc/X11/xorg.conf.d/20-keyboard.conf, these configs:
Option "XkbLayout" "pt"
Option "XkbModel" "pc105"