This source is not very trustworthy. No one usually uses 24-hour format in a few places not noted here, such as Holland, a decent-sized European country that many people have heard of. But they are not shaded in this chart. So I wonder how many others are also not shaded.

In many European countries, the 12-hour format is used in speech, but not in writing. You'd say "Let's meet at seven", but you'd write "Let's meet at 19:00". The context being software, I'd say it's a pretty solid claim to say most countries use a 24-hour clock.