A fire extinguisher is mandatory in Belgium, Poland, Turkey and Greece.
First aid kit
A first aid kit in cars is mandatory in Belgium, the Czech Republic, Germany, Austria, Slovenia, Slovakia, Hungary, Croatia, Turkey and Greece.
Be sure to bring a set of safety vests with you in the car. This is mandatory in most EU-countries. In Luxembourg and Italy, it is even compulsory to have a safety vest for all passengers in the car. Buying a set with multiple vests is the easiest option. Then you'll always be good.