There is a mechanism for births.
Have a friend born in Paris in 1930 to British parents. Her birth was registered at the mairie under French law, and also notified to the British Embassy in Paris at the same time.
Otoh, my uncle was born in the Somme in 1922 to a French mother and a British father who worked for the Imperial War Graves Commission as it was then. His mother had no right to vote nor to pass her nationality on to her son. By the time he was 21, he was a first officer in bomber command. He's completely under the radar. His father accepted the view of his mother as he was a foreigner, and his mother, conscious of her perceived status being a relative of Madame Pompadour wasn't bothered by post-revolutionary bureaucracy. He has a British Passport.
His birth is registered at the Mairie in 'his' village.
His first and second marriages are registered in the UK, as was his divorce. My aunt"s ashes are buried in his parents" grave in the village. But, even though the Mairie had to open the grave and dig a hole for the casket, and knew her, as far as officialdom is concerned he isn't a widower, but single, never married.
Ancestry, being an American site isn't particularly strong on this type of thing. I used to have full world wide access. When I didn't and looked on the French site I could find his mother, and him, but never on the world access. I contacted ancestry and they completely refused to accept this because it doesn't fit with the way they see the world.....
Paypal used to be like this, I have bank accounts and credit and debit cards in both the UK and France. They told me that this is illegal. They(ve got better.