Obviously it's not the lettuce that is commonly used in meals.
It's in the same family(?) as that lettuce, the specific 'strain' I mean is called 'wild lettuce' and isn't cultivated or anything since it's not used for anything (so not in meals either). But it's supposed to have some opiate-like effects. I never bothered researching the stuff very thoroughly though, but I'm almost sure this 'wild lettuce' that I mean has some psychoactive effects. How strong they are I do not know, if they were worthwhile I'm sure the plant would be more commonly known amongst drug(opiate especially)-users. I imagine kratom being like a horse tranquilizer next to this stuff, I could be wrong, but I still cba to research it any further.
Yeah I know what you mean by nettles, just figured you might've been mistaken since I never heard of psychoactive nettles either, just the lettuce thing. But I guess you're not, in which case I'm not gonna be much help since I've never heard of such a thing either (and frankly I would be surprised if they are illegal in France, the only 'medical' application they have is to combat rheumatism AFAIK).