I’ve gone through exactly the same situation as you, I have my Class 1 but it’s true in France/Spain you have to have a FIMO or FCOS, the difference in the above is dependant on how long you’ve had your HGV licence.
FIMO course - one month in length, three weeks classroom based including your basic hazardous goods - one week driving.
FCOS - three day course classroom and practical all in Spanish