Hi,
If you read the installation instructions in the documentation you'll see the list of files the module uses.
You'll probably have to create Italian files for the e-mail files. I think you maybe then put them in email/it (IT for Italian). It shouldn't be any different from adding e-mail templates for any other module as the Back In Stock Notifications module simply uses the standard zen_mail function.
All the best...
Conor
ceon
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