The easiest way to do this is:
1) Create a skin that shows nothing except the main content area. No header, menu, footer, etc...
2) Create a new page in your master site (Italian), set it's Parent to Root, Uncheck Include In Menu, and set it's Skin to the skin you created in Step 1.
3) Go to Administration > Add/Edit Pages and move the page you created in Step 2 to the top of the list. To do this click on the page and then click the UP arrow icon until the page is directly below the first page (Itawan) in the list. Now, select the first page (Itawan) and click the DOWN arrow icon.
This will make the new page the default page and visitors to your site will see the flash intro that you put on the page. You don't have to change anything with the other sites or RelatedSitesMode.