To ride in Europe, you do need an IDP by law.
Italy, Germany and Spain are progressive European Union countries. You must show that you have an IDP – International Driver’s Permit. By law you must now show both a driving license and an IDP for any non-EU resident to ride or drive in the EU, and to rent your motorcycle. Once on the bike, you will probably never use it again, but should an incident occur, this paper will get you out of a fine or other problems. It is recommended to get this 1 or 2 months before your travel date. Depending upon the state and issuing body, the IDP is good for 6 months to 1 year and usually costs around $20-$35. In the USA your local AAA office is an easy place to purchase one, just bring some passport sized photos with you and you can fill out the form online or in office.
https://www.aaa.com/vacation/idpapplc.html for USA residents.
https://www.caa.ca/international-driving-permit/ for Canadian residents.
https://www.internationaldrivingpermitsonline.com.au/ for Aussie residents.