Suspecting a mess with the snaphots (made automatically by grub-btrfs), I made a new one of the current system (with the last version of kernel) and I removed all others snapshots. Then I did update-grub and I rebooted.
The weird additional entry is no more present so the problem is solved (even if I cannot explain what happened).
Edit: May be it is linked to the fact that I restored a previous snapshot when I saw the US keyboard problem. The problem started when I applied again the update.