Hallo Herve5
Thank you for posting your last piece of information. 
I’ve just upgraded my main system from 16.04 to 18.04. It was a fight (the bootloader broke).
When I had 18.04 up and running the system registered my keyboard as “QWERTZ” (with some special signs) - but - the logon screen still thought it was a standard “QWERTY” keyboard. Once running, the keyboard was then recognised as “QWERTZ”.
I applied a “fix” that I had previously found. It did not work. I then applied your:
(your account):~$ sudo localectl set-x11-keymap “fr” “” “oss”
replacing “fr” with “ge” for German. This solved the logon keyboard problem. 
I had one more keyboard problem though, the sudo password (terminal, synaptic, etc.) still thought I was running a “QWERTY” keyboard. I solved this by entering user-accounts and resetting my password.
Merci 
.