for a list - check somewhere like AA or RAC in the UK because they expect members to drive on the continent, or US Military guidance for similar reasons. The RAC: https://www.rac.co.uk/drive/travel/country/france/
I drive through many countries and carry the following, each required by one or many of the usual western / central European countries:
- Hi-vis reflective vest for every person in the car. If you break down or have an accident everybody out of the car must have one on. In France it must be put on in the car before stepping out on a highway.
- Spare set of light bulbs - France
- Fire extinguisher
- Warning triangle - check each country for distance behind to put
- First aid kit - Germany
- Environmental stickers if you intend to drive in built up areas of France or Germany
- Registration online for driving in built up areas of Belgium and UK
- IDP in many countries for each driver
You should also understand the rules of the road - different in most countries, signage - different in many, national speed limits,
understanding of implications of ZTL - Italy (now a few in France), if you don't learn about any different sign understand the meaning of the yellow diamond.