I remember reading people talking about the fact that the use of tritium is forbidden in France (automatic watches used to have tritium paint to glow in the dark; now they mostly use phosphorescent paint instead), mostly because it is radioactive.
I've been trying to find anything official (laws, documents, etc.) online and so far I wasn't able to find anything.
Is there any official law that prohibits the use of tritium in France?
Yes: it's the French decree 2002/460 of 4 April 2002.
It does not mention tritium by name, but this article explains why it is now prohibited:
The French authorities have just published decree 2002/460 of 4 April 2002 concerning general personal protection against the dangers of ionizing radiation. Having as its aim the transposition into French law of European directive 96/29 Euratom of 13 May 1996, which lays down basic standards concerning the health protection of the population and workers against the dangers resulting from ionizing radiation, the decree prohibits any intentional addition of artificial and natural radionuclides in consumer goods, as well as the import and export of such goods. Exceptions may be granted to ensure the safety of consumers (for example for diver's watches). No exceptions are made however for decorative pieces (jewellery and watches in general).
It follows from these provisions that the use of tritium and other radioluminescent products of the same type for the hands and pointers of watch dials is now prohibited in France. It is now necessary by law to use other luminescent solutions. Source Dated 23 May 2002