Kali 2021.01 – add keyboard layout via terminal

setxkbmap

setxkbmap -layout <countrycode>

e.g.:
setxkbmap -layout us
setxkbmap -layout de

localectl

sudo su
localectl --no-convert set-x11-keymap <countrycode>

e.g.:
localectl --no-convert set-x11-keymap us
localectl --no-convert set-x11-keymap de

reboot