Current session : 
$ sudo setxkbmap be

Permanently : 
- Go to Settings > Region and languages > Input sources
- Click on the + 
- Click on the three vertical dots
- If you type "Belgian" in the search bar, you will only see "German". Yes, really. I don't know if some history nerd made a joke or something, but don't be afraid to click on it.
- You should now see all the variants of the Belgian keyboard layout.