The 12h and 24h system is essentially the same thing, no? In my country we use a 24h system in writing, but mainly a 12h system in speech.

In Punjabi we have divided day into 8 parts, with each 2 parts have names like 1st pair (3am to 9am), 2nd pair 9am to 3pm) third pair 3pm to 9pm & final pair 9pm to 3am.

So, we usually say, 8 o clock in third pair means PM, or 8 o clock in 1st pair means AM.

I think most of the world does that. I don't believe the claims that almost nobody uses the 12h format. At least not without a very thorough survey of the many ways many cultures use time.

In Brazil we'll say things like "The party is at 8 da noite", where "da noite" means "at night". When context is obvious, we often ommit that, so I might say "We will have lunch at 1 one clock" meaning 13h because it is obvious that no one has lunch at 1am. We will generally use 24h time in writing because we cannot control context, and in other situations where ambiguity cannot be avoided otherwise.

To me the main advantage of 24h time is that it is easier to mentally calculate time, so for example 3 hours after 11:00 is just 11+3, which is 14:00h.