I just tried switching my Mac to Italian and after restarting checked my About screen to see if I could recreate your problem. I failed. It showed up correclty, but in my case it still showed the English string translation for the model size and year (instead of 13 pollici, Meta 2011 I got 13-inch, Mid 2011).
This makes me think there is a string translation error somewhere else. I'd be curious if other Italian MacBook AIr users have the same issue. If you change your system preference, language & text, from Italian to Enlgish I'm guessing it'll still show the same problem.
Do you also own a MacBook Pro? Supposedly the serial number is linked to the Mac model. Did you ever reinstall the OS? If every default install had this issue every Italian user would be running into it. Might check the Italian forums to see if this is a known issue, or your setup is unique.
Sorry I couldn't be of more help. I'm guessing the error of identifying the wrong Mac is just a cosmetic issue, but I agree it is annoying.
You might be able to hack the string to show the correct model, but I don't the correct steps, and for stability I'd recommend not changing it. I think you should contact Apple. Maybe you'll get a swapped out MacBook if they can't resolve it.
http://www.youtube.com/watch?v=rh11Ml3QYEY
http://www.insanelymac.com/forum/index.php?showtopic=273479