Any translation files placed here will be deleted when you update Storefront.
Put any custom Storefront translation files in your WordPress language directory: /wp-content/languages/themes/storefront-it_IT.mo.
Alternatively you can put translations in your child theme: /wp-content/themes/your-child-theme/languages/it_IT.mo.
If you would like to help out translating Storefront, please contribute on our translation page at Transifex