Not French but francophone so maybe it is an English thing, because I heard so many anecdotes from Anglophone relations about their imaginary friends growing up, and mon doux jesus I tried to have an imaginary friend, but sadly found myself impotent in that regard.
It seems to be a swedish thing, but sweden is loving US/UK culture so maybe we hear about it but doesn’t have them.
For what it’s worth, I grew up there and never met/heard of anyone having an imaginary friend, but I do remember it from TV. Hmm, maybe a german film? What’s about you, germans?