As anybody answered, I decided to investigate by myself.
And I found that in the default.xml in share/keys, the 5 key (ZoomPage action, the only one which worked for me) is defined twice:
<bind key="5" action="ZoomPage" display="true"/> <bind key="KP_5" action="ZoomPage" />
Other numerical keys are not defined twice. So, KP_5 means the 5 key on the keypad. I don't know why other numercial keypad keys are not defined... Ok so I defined twice all the keys from 1 to 6 in default.xml, and it works now.
Now, for the "normal" zooming keys, it was not working at all. And I found out the trick: on a french keyboard, the digits are the reverse from a US keyboard: you have to Shift before. So I added modifier="Shift" in the key definition and now it works.
The only drawback is that the label in the menu is not for exemple '1' but 'Shift+1'.
Hope this helps.