Italian language dictionary for Safari
Safari will use the language(s) that you have installed when you installed you operating system and it will try to automatically determine the language you’re typing in (provided a dictionary exists). Since you’ve mentioned Safari, I assume you are having a lot of red underlines when typing in forms in Italian. 

Make sure you have Italian in your system (do you use the operating system in italian?) Do your applications open in Italian?

If you right click on a textbox in Safari and open the Spelling and Grammar palette:


You will get this:



Notice the “Automatic by Language”. I have italian there, and in fact, that list is ordered by the order of the languages in Language &amp; Text preference pane. 

Now if what you want to do is modify the built in Dictionary app to support other languages, I’ve done that in Leopard (added spanish) and I haven’t tried to do it again in Snow Leopard, but here is a very nice tutorial on how to do it.