The vast majority of European countries at least [0]. Every car needs to have one, their contents is standardised and since they're produced in huge quantities, they're hilariously cheap.
First aid training and an exam are also mandatory for getting your driver's license in many countries.
Check out the link, it also has color coded maps for first aid kit, fire extinguishers and high-vis jacket requirements.
The first aid kit also usually means required first aid training as part of getting a driver's license. However I don't know if any countries require refresh sessions, so most people probably forget all except the basics.
In which country is it mandatory?