I've uploaded the pdf of the guidance from the French government (thanks to
http://www.thisfrenchlife.com/thisfrenchlife/2008/05/warning-triangl.html).
There is detailed guidance for drivers, which among other things says that the gilet must be CE-marked.
The guidance for cyclists is in a separate section and says:
ET NOS AMIS CYCLISTES ?
Obligation pour tout cycliste circulant hors agglomération, de nuit ou de jour si visibilité mauvaise , de porter un gilet
rétro-réfléchissant à partir du 1er septembre 2008. L’obligation s’applique aussi au passager éventuel.
There is no reference to the EN - I don't know whether this reflects the text of the original réglementation. However given that in the rest of the document the authors have specified the need for equipment to conform to European standards, there may well be a reason why they are not mentioned here. On the other hand I'm not sure I'd want to argue the point at the end of a long day. As it is, my
Decathlon gilet is CE marked as conforming to EN471-03 so I'm alright Jack.
My gilet is very similar to other cycling gilets, and I suspect that most cycling high-vis gilets available in the UK meet the EN.
EDIT: OK if anyone wants to get really geeky about this the obligation comes from a decision taken at a meeting of the Comité Interministériel de la Sécurité Routière on the 18 February. The press dossier is here:
http://www2.securiteroutiere.gouv.fr/IMG/pdf/DP_cisr_13_02_08.pdf
The important bit is on page 12.
Interestingly (or confusingly, if you're the original poster) the wording of the decision is different:
Les cyclistes sont très peu visibles de nuit, tout particulièrement hors agglomération en l’absence d’éclairage public.
Le Comité interministériel de la sécurité routière décide de rendre obligatoire le port d’un gilet rétro-réfléchissant par tout cycliste de nuit hors agglomération à compter du 1er septembre 2008.
Les cyclistes qui ne porteraient pas de gilet seront passibles d’une contravention de la deuxième classe.
.. no reference to poor visibility.
The requirements for drivers stem from a European directive (hence the reference to European standards) while the requirements for cyclists are purely national so it is possible that the legal requirments are indeed different. But it's probably still simpler to carry a gilet for emergencies.