I'm in Belgium (french part) and I cannot find the proper keyboard layout in Settings > Region and Languages > Input Sources. the closest I found is the french (AZERTY) one but for the symbols such as ! and _ it's not the same keys... what can I do to solve it ? Thank you.
You may have hit this issue. In that case you can generate a Belgian locale, e.g.:
sudo locale-gen fr_BE.UTF-8
Then you should be able to click French and find some Belgian layout options in the sub menu which opens.