It needs to be added to /usr/share/sddm/scripts/Xsetup
Something like this, what i use in my case:
setxkbmap -model pc105 -layout us,ro,es -option grp:alt_shift_toggle
So it should be generated at install, or edited manually by the user.
More about this here
https://wiki.archlinux.org/index.php/SDDM#SDDM_loads_only_US_keyboard_layout
and here
https://wiki.archlinux.org/index.php/Xorg/Keyboard_configuration#Using_setxkbmap
With this, any sddm theme should show the keyboard layout at login, including our Breath2.
Hope this helps. 