Alessandro,
I’ve been almost in the same situation, it’s hard to find offers but as hiower said, best way is to contact the companies directly. So far, I couldn’t find a platform like coroflot or productdesignhub for internships in Europe. However, there’ll be some offers on this sites from time to time. design directory and dexigner should be helpful. If you are interested in Germany, I’d suggest you to take a look here http://www.design-report.de/DRMainJobsAngebote.asp?th=Angebote
There are new offers almost everyday, almost all of them are in German, but you can use google to read them and apply in English.
Good luck!