@sven
The solution was to declare the existing french cosmo layout in the evdev.xml file. I added :

<variant>
   <configItem>
     <name>fr</name>
     <shortDescription>fr</shortDescription>
     <description>French (Cosmo Communicator)</description>
     <languageList>
       <iso639Id>fr</iso639Id>
     </languageList>
   </configItem>
 </variant>

in the "planet" section as explained here : https://gitlab.com/ubports/community-ports/android9/planet-cosmocom/planet-cosmocom/-/issues/3

Then the Cosmo communicator layout appears in the variants of French in the physical keyboard menu of UT. Didn't touch to the layout file at all.

Thanks both of you for the tips. Works perfectly now.

@moncon Worked perfectly for me ! It is soo easy !
I had more difficulties with the UBports installer on other devices.
Now I have a new toy to go along my daily driver.

I have the same timezone and the same problem with +2hrs. If I use the workaround, the app displays the correct time for my events. The problem is I can't create an event from the phone at the right time because it will be +2hrs on Nextcloud.
So far I haven't found a way to make it work properly both ways

@mou-du-gnu I pushed my exploration of UT on Cosmo with a french keyboard further.
The keyboard layout works perfectly on UT and apps.
I tried to install a couple linux apps in a Libertine container and suddenly I'm back to sort of a qwerty layout. The keyboard layout indicator in UT switches to "En" (English Cosmo communicator, UK) wich explains the issue. I haven't found a way to revert to a French layout.
As soon as I go back to an app in UT, the layout is correct again.

@sven
The solution was to declare the existing french cosmo layout in the evdev.xml file. I added :

<variant>
   <configItem>
     <name>fr</name>
     <shortDescription>fr</shortDescription>
     <description>French (Cosmo Communicator)</description>
     <languageList>
       <iso639Id>fr</iso639Id>
     </languageList>
   </configItem>
 </variant>

in the "planet" section as explained here : https://gitlab.com/ubports/community-ports/android9/planet-cosmocom/planet-cosmocom/-/issues/3

Then the Cosmo communicator layout appears in the variants of French in the physical keyboard menu of UT. Didn't touch to the layout file at all.

Thanks both of you for the tips. Works perfectly now.

@moncon For me the layout is good for all the keys but the "m" one (output is "ù") and Fn key doesn't work. I think it is surely a keyboard mapping problem. I don't have it with the linux boot.

@moncon How did you manage to use the Cosmo keyboard properly ?
On my French layout, I have a problem with the "m" letter and all caracters accesible by Fn+key.
Only thing that prevents me to try Cosmo as my daily driver.
Thanks again for the install.

@moncon Worked perfectly for me ! It is soo easy !
I had more difficulties with the UBports installer on other devices.
Now I have a new toy to go along my daily driver.