I finally figured it out.

This is what I did:
I clicked on Languages -> Italian -> Site -> Local resources -> Desktop Modules -> Admin -> Security -> App_LocalResources and selected the file that on the tree appears as User.ascx.

What really happens is that the system hides part of the name from you and the file's real name can be seen at the top of the page.
If you scroll at the beginning of the page it shows you the language, the path and the real file name which, to my surprise, appeared to be, as it should be, User.ascx.it-IT.Portal-8.resx.

When I tried to select the item that appeared to be the correct file, namely User.ascx.it-IT.Portal8 to my surprise I realised its real name to be User.ascx.it-IT.Portal-8.it-IT.Portal-8.resx

These files have probably been renamed during the upgrade and are no longer used by the assembly. You can copy paste the correct translations to the new file but they were not imported automatically by the system.

Something has indeed changed with respect to the previous versions and that is the way the system views the resource files.

Mistery solved.

Way to go Edoardo!

And thank you for posting back your solution :)

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