My mum is a (now retired) primary teacher, 45 years so a lot of British kids have passed before her. We live in France, and she has consistently noticed how confident, articulate and polite my kids French friends are, from an early age. Several reasons I can think of:
Social politeness is utterly drummed into them from day one. Even the youngest children are expected to say ‘bonjour monsieur/ madame’ to any adult they encounter - whether it’s a neighbour or someone they don’t know. I remember going on a hike and meeting a family coming the opposite way - all the kids, even the youngest trilled ‘bonjour Madame!’ as I passed them 😂 on the track. Shyness is not an excuse. Parents get bollocked by older people if their children don’t say ‘bonjour’ correctly 🙄
At school they have to learn by heart and recite poems in front of the class, from a really young age. They do this regularly throughout school. As they get older, they will work towards a grand oral exam - where they make a prepared presentation to a jury of examiners, and are grilled on it. The ability to talk fluently, from memory, and to defend your ideas is considered a very important skill and, again, it’s not optional- every student has to do it.
Discipline is quite strict. Smacking is not illegal and my dc have told me of their friends being smacked, having their ears boxed, being slapped. My French colleagues have smacked their kids and don’t see any problem with it.