As with any issue of damp, you need to trace where the source of the problem is coming from and tackle the problem from the root. There are many causes of damp. If the source is under the floorboards, some or all of the floorboards may have to be removed. However, damp problems can emanate from things like leaking pipes, chimneys, etc., and even lifestyle issues can have a bearing, such as drying clothes indoors, steaming food, etc., and not airing the property properly. French properties typically have no damp-proof course or air bricks for ventilation. In my old French house there were multiple issues causing damp problems. I had to pull up all the floorboards which exposed bare earth below before contracting builders to install a waterproofed concrete floor. This helped, but I had to have a French trench dug behind the property to help with water drainage and have the rotting windows and doors replaced to prevent water ingress.