Do you mind me asking where you are?

All my experience has also been using 12h time in casual situations in 24h countries. I'm curious of a country that doesn't, and whether that's true of the whole region.

Poland. It seems to be more of a generational thing than regional. People over the age of 70 seem to use 12h. Younger ones use 24h.

Ireland, New Zealand and Finland used 24 casually when I was there

Ireland uses 24h clock in writing (including casual writing, text messages, etc.), and it wouldn't be at all unusual for personal digital clocks to be set to 24h clock, but I'd be surprised to hear it in speech.

I've never heard anybody from a 24h country use 24h unless it's regarding situations where there cannot be any ambiguity. You either live in an exceptional place, or interact with exceptional people. World you really say "At fifteen o'clock" instead of "At three o'clock" or "Three in the afternoon"?

Yes.

You obviously haven't been to France.