“Cliff Richard to me is perfect, and I do love him,” writes the 16-year-old Jane Birkin, stuck at boarding school on the Isle of Wight with only her diary and a stuffed toy called Munkey for solace. She has, it is true, got hold of a contraband copy of the book Peyton Place, but feels “very coarse and common” whenever she dips into it. Such is the inner life of the girl who, within seven years, will release a pop song so smutty that it will be banned by the BBC and condemned by the Vatican. “Je t’aime … moi non plus”, written by her lover Serge Gainsbourg, became a global hit in 1969. Even now, 50 years later, Birkin remains associated with a particular kind of arty transgressiveness, not to mention the very expensive Hermès handbag that bears her name.

This is the first volume of her diaries, which came out in France three years ago. One wonders why it took so long for them to be published in English, Birkin’s native language. Could it be because, after a promising start describing a postwar British upper middle-class childhood, all faithful family retainers and woolly jumpers, they don’t make much sense? The moment that 19-year-old Birkin swaps Chelsea for Paris, where she will live for the rest of her life, the prose descends into what can only be described as word soup. Here’s Birkin describing the after-effects of recording another Gainsbourg song, “The Ballad of Johnny Jane”: “I had short hair to play Johnny Jane – Serge didn’t want me to cut my hair because he didn’t want me to look the same in other films.” So … she had a crop, a wig, or something else entirely?

Gainsbourg sounds awful, but that’s not the point. He also sounds completely flat, made up on the page with whatever words Birkin happened to have to hand. For instance, we are told that although he only had a bath every three months at most, he was nonetheless exquisitely clean. “He didn’t sweat and I never noticed the slightest odour,” marvels Birkin and yet, 20 pages later, a sudden summons to hospital with heart pains means that Gainsbourg has to quickly plunge his feet into the bidet “so they wouldn’t pong”. It feels pedantic to point out that quite a lot of people, even in the 1970s, worked on the assumption that they could take their shoes off without clearing the room.

All in all, the effect is rather like reading Gertrude Stein, the wealthy American art collector who set up home in Paris 50 years before Birkin. Her most famous book, The Autobiography of Alice B Toklas was a disguised memoir, just like Munkey Diaries. But the difference here is that Stein set out determinedly to test the limits of language. With Birkin there seems to be no such intention. Either the original French and English (Birkin tells us she used both in her diaries) doesn’t make sense, or this particular English translation doesn’t. In February 1970 Birkin announces: “This is a photograph taken about a month ago of Serge and his ma, and aren’t they sweet?” But there are no photos. If it were Stein writing you would put it down to a playfully deliberate frustration of readerly desire. With Birkin it feels like she just hasn’t noticed that something crucial is missing.

Anyone hoping for an insight into Birkin’s career will be particularly disappointed. One minute she’s a schoolgirl trying on mascara in the ladies’ lav at Peter Jones, and the next minute she’s at a party given by Roman Polanski where she meets the composer John Barry. He wants to marry her, gets her pregnant and then refuses to sleep with her again. So she goes off to Paris, meets her Svengali and becomes a film actor and recording artist. Gainsbourg tells her she is “porky” with nonexistent breasts. She in turn writes a letter informing him that he has the eyes of an “electrocuted toad”.

On and on they go, drinking themselves into unpleasantness, dreaming up new ways of hurting each other. She throws herself into the Seine in front of him, while he, on the point of dying from a heart attack, tips off the newspaper France-Soir to make sure they send a photographer. Reading these diaries is like being trapped at a particularly demented piece of performance art, where the actors are clearly having much more fun than the audience.

• This article was amended on 23 August 2020 to correct a misrendering of the title of the song “Je t’aime ….moi non plus”.

• Munkey Diaries is published by Orion (£20). To order a copy go to guardianbookshop.com. Delivery charges may apply.