What I need to know is, in which files are the keyboard settings located.
Obviously, mabox handles that different than manjaro does (since this problem does not occur in the manjaro edition of pacbang - which is no longer listed in the community versions available for Manjaro).
Mabox is based on Manjaro and it handles that the same.
You can change keyboard layout from cli with command (for example):
setxkbmap us
See man setxkbmap if you want to know more.
I checked solution proposed by me with first answer (with fbxkb) and it works perfectly fine.
I even installed xfce4-panel and it also works perfectly fine with it.
I have no idea why it does not work for you, and quiet frankly I won’t spend any more time on this.
Currently we have it almost preconfigured by default.
Check ~/.config/openbox/autostart and configure it to your needs.
Lines: 23-25
## Switch keyboard layouts by Alt+Shift
## Example below: US an Greek layouts
#setxkbmap -layout "us,gr" -option "grp:alt_shift_toggle" &
set desired keyboard layouts and uncomment last line.
If you like graphical indicator in systray, there is gxkb preinstalled.
It can be enabled in autostart by: Menu → Mabox Config → Autostart → GUI Editor.