Source: https://superuser.com/questions/1147320/how-to-change-keyboard-layout-in-i3

Execute this commands

setxkbmap -layout us,de
setxkbmap -option 'grp:alt_shift_toggle'

Or put this on your i3 config file (~/.i3/config, ~/.config/i3/config…)

exec "setxkbmap -layout us,de"
 
# optionsl
exec "setxkbmap -option 'grp:alt_shift_toggle'"