The French government has decreed that smoke detectors must now be used in French properties. Apparently every property must have at least one smoke detector installed. Ideally the property will have smoke detectors fitted in each corridor and main hallways. The following page stipulates the regulations.
Of course it goes into great detail as to what type of detector is OK. Basically if you buy them from any shop in Europe you should be fine as they will have to conform with the EU norms. It looks like you should let your insurer know what you have fitted and where too. There are penalties to deal with properties that burn down without the smoke detectors fitted (so you’ll have to prove they were) and liabilities if 3rd party damages are caused too!
Probably the best “plain english” description of the situation is to be found here.
The smoke protector pictured costs slightly more than the most basic, it’s positioned in the main living area and when it is triggered all the other smoke alarms in the house go off at that same time. Not rocket science.