Mains powered fire detectors are a big no in France with differential breakers on every circuit, better to have a couple of battery powered ones. I stayed with friends in Arcachon over the summer and the coffee machine went up in flames in the kitchen. Both the circuit’s 30mA breaker and the 500mA breaker went cutting off power, and giving us an audible warning, at that point there wasn’t enough smoke to set off a smoke alarm in the main room.
The wood walls are some of the least inflammable things in that room which is now strew with paper. I could have painted the wood and might do if we fancy a change of colour, jfletch. I could also skim it with wood filler and then use a fiberglass renovation paper and paint.
Or use polystyrene backed plasterboard as I first suggested, it’ll give you a smooth surface you can paint any colour you want or paper, and insulate better than cavity wall filling ever will without humidity problems.