We have resolved the situation in this way:
amministratore@ip-xx-x-x-xx:~$ locale -a
C
C.UTF-8
en_US.utf8
it_CH.utf8
it_IT.utf8
POSIX
C
C.UTF-8
en_US.utf8
it_CH.utf8
it_IT.utf8
POSIX
I also realized that the certificate print date was due to the setting "User's Language Date Format" 
But now we have our week names in the chosen language!
Thank you for your assistance!