You can use setxkbmap:

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

Just change the keyboard layouts us,de to what ever you prefer.
If you want i3 to run these commands on startup, just add them to your config file "~/.i3/config" and prepend "exec".

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