localectl-examples.sh

# check the current layout settings

localectl status
  # System Locale: LANG=en_CA.UTF-8
  #     VC Keymap: ca-multix
  #    X11 Layout: ca
  #     X11 Model: apple
  #   X11 Variant: multix

# the command to set keyboard layout is 
# localectl set-x11-keymap LAYOUT [MODEL [VARIANT [OPTIONS]]] 
# so we need to figure out what to put 
# for LAYOUT MODEL and VARIANT for my keyboard

# to list all available layouts
localectl list-x11-keymap-layouts 
# from the output of the above command 
# I figured that LAYOUT=ca in my case

# now there is a command to list available models
# since I was interested in apple, I added a grep
localectl list-x11-keymap-models | grep -i apple
  # apple
  # apple_laptop
  # applealu_ansi
  # applealu_iso
  # applealu_jis
 
# from the above I figured that for me MODEL=apple 
# would be a good first guess.
 
# Now we need to figure out the value for the VARIANT 
# for LAYOUT=ca
localectl list-x11-keymap-variants ca
  # eng
  # fr-dvorak
  # fr-legacy
  # ike
  # multix

# I tried using fr-dvorak and it was a disaster, 
# it completely messed up my keyboard, 
# using trial and error approach I realized that VARIANT=multix 
# for my Apple keyboard with numeric keypad.

# So, finally I have all the ingredients 
# to set my layout as follows
localectl set-x11-keymap ca apple multix

# and use cmd+shift+e to exit i3 and then 
# login again and enjoy a nicely working keyboard in i3))

## UPDATE:
# I needed the left option/alt key to behave as altgr, it is just easier to type the curly and 
# square brackets on the apple keyboard, to achieve this you would need to add the 
# following argument to the localectl command 'lv3:alt_switch' as below:

localectl set-x11-keymap ca     \
                         apple  \
                         multix 
                         
# I used the following to swap left and right alts
setxkbmap -option lv3:lalt_switch -option lv3:ralt_alt

# to save the above setxkbmap options for switching alts, modify /etc/default/keyboard as follows:

cat /etc/default/keyboard 
'''
# KEYBOARD CONFIGURATION FILE

# Consult the keyboard(5) manual page.

XKBMODEL="apple"
XKBLAYOUT="ca"
XKBVARIANT="multix"
XKBOPTIONS="lv3:lalt_switch,lv3:ralt_alt"

BACKSPACE="guess"
'''

# To persist the switch of the left-right alts in i3
# I had to add the following to the ~/.config/i3/config

exec --no-startup-id setxkbmap -option lv3:lalt_switch -option lv3:ralt_alt