The names of the mailboxes default Exchange (Inbox, Sent Mail, etc.) are created in the language used for the first access to the mailbox.
To override this problem you can do that from powershell on Exchange server:
Set-MailboxRegionalConfiguration -identity -LocalizeDefaultFolderName:$true -Language it-IT
Otherwise you need to go to:
OWA -> Options -> Settings -> region and here in addition to select the language you put the flag in the option Rename default folders with a name that matches the language specified.
[Source page http://sistarelli.com/2015/06/06/rinominare-le-cartelle-predefinite-di-exchange-2013/]