In the admin area go to
System->Localisation->Languages
and add a new language with the following data
_______________________________________
Language name: Italian
Code: it
Extension:
Locale: it.UTF-8,it,italian
Directory: italian
Filename: italian
Status: Enabled
Sort Order: 0
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Now you can edit the settings of your store via
System->Settings-Your Store (Default)
after choosing the store select the "Local" tab
and change the following informations
Language: italian
Administration Language: italian

Demo: https://dochoixemay.online