Set Layout with X Display

Set layout to British English.

setxkbmap -layout gb

Or Polish with:

setxkbmap -layout pl

Language	short
Polish	pl
Serbian	rs

Set 'alt + shift', as the command which cycles through the British English, Polish and Serbian keyboard layout.

setxkbmap -layout gb,pl,rs -option grp:alt_shift_toggle

Alt_GR

Remap, e.g., the right Windows key, to Alt_Gr.

    key <RWIN> {[  ISO_Level3_Shift    ]};

Copy your keymap, e.g. if it's polish-1, then:

cp /usr/share/kbd/keymaps/i386/qwerty/pl1.map.gz /usr/share/kbd/keymaps/custom.map.gz

Then change that map:

sudo -e /usr/share/kbd/keymaps/custom.map.gz

Change the default keyboard mapping to the custom map:

echo 'KEYMAP="/usr/share/kbd/keymaps/custom/custom.map.gz"' | sudo tee /etc/vconsole.conf

Reboot to have changes take effect.