Storefront will look in this directory for translations as a fallback. It is however recommended that you use the global WordPress language directory and install your translations like so: /wp-content/languages/themes/storefront-it_IT.mo. That way they will not be lost or overwritten during Storefront updates.
Alternatively you can put translations in your child theme: /wp-content/themes/your-child-theme/languages/it_IT.mo.
You can read more about installing Storefront in your language here.