I often go for a walk in a sensitive natural area (ENS) on the heights of Beausset (Var) and I have observed an Euphorbia myrsinites there on several occasions. I saw two plants there: one on a terrace (probably a cultivar) and a second plant which grew by itself in the ENS car park.
My book “Flore de la France Méditerranée continental”, which mentions all the plants present in this region, including exotic breakaways from cultivation, does not mention this species.
So, here is my question: Is this an oversight on the part of the authors of the book or is this type of case in this species too isolated for the species to be considered naturalized by the authors?
A printed book is always going to be out of date, perhaps a year before it is published - because of the time it takes to prepare for publication.
Always good to have an online source to bridge the gap. Something like iNat for example.
There are a few observations of this species from all over France on iNat, so this may help decide. You could check through them and see if they are obviously cultivated or not. If not, it’s probably fair to say that they are starting to be naturalized. I agree with @dianastuder : books are a great resource, but especially for things like introduced species, they are always going to be at least a little out of date.
It all depends what’s en vogue with the gardening crowd at any given moment. Probably a plant that has been popular only recently as a garden plant. It’s non - native in France, you can add a field to the observation.