Multisite is the deep-end of WordPress. If you're not already experienced in debugging and know how to build out test sites and localhosts you really don't want to be playing here :( It's hard. We know. It's also really dangerous if you use plugins without vetting them first.
Moreover, in these last two days, all the three sites I built with WP MU have problems: only the main site is visible, and the secondary ones are horrible, because the css and everything else isn't working (only rough text is visible in http://ilcavallobianco.com/www/en/...).
I checked both index and htaccess.
THAT means your .htaccess isn't being read properly, which is a server level error.
You'd want to look at your php error logs to see what that reports (see? I said it's hard stuff).