I don't speak French - if I were to just judge based on the music, I would say it sounds like what would happen if Leonard Cohen turned French and funky. The music alone would probably bring the album to at least a 3, if not a 4.
However, the album cover provides cause for caution: Jane Birkin as a young, topless girl clutching a doll over her chest. She was 25 when the album was released, so the pedophilia is suggested, not actual...until you read the translation of the lyrics. In a nutshell, while driving one night, the narrator hits a young girl who is riding a bicycle: Melody Nelson, who we learn is not quite 15. He becomes obsessed with her (and whether she's a virgin - saying "Oh! Melody / You don't know what love is / You told me that yourself / But is everything that you say true?" and then "Oh! Melody / If you lied to me I'd do / Some crazy things / I don't know what I'll do to you"). He takes her to what sounds like a pretty seedy hotel and, well, "Slowly, I embraced Melody." Ugh. Spoiler alert: she dies in a plane crash and he is left poetically pining for her.
Maybe this album is shades of Lolita - you're supposed to recognize that the narrator is a horrible person. At least I hope so, because if he meant it...ugh. Double ugh. Reading up on Serge Gainsbourg, it sounds like he was a provocateur - and hopefully provocation was his aim - but it's just too hard not to think of this album as a paean to pedophilia. The seductive musical style makes it even worse. The ick is too strong, and I move on.