Good question. By the way, Germans and French people at least do use 12hr time, but only colloquially.
When talking to someone, you’d say things like (literally translated, obviously) “let’s meet at five”, or “I saw them at around 5 in the afternoon”.
Using 24h time when speaking is common as well, but tends to emphasize formal time or preciseness. Something like “they are open until 18:30”, “the movie starts at 20:45”, or generally if there’s not enough context or a risk of getting it wrong.
So conversely 12h time can also carry a subtext or impreciseness, e.g. if I say “see you at five”, then often it’s around five, but if I say “see you at 17 o’clock”, I more likely mean 17:00 sharp.
Markers like “AM/PM” don’t exist, at all. When using 12h time, it’s either clear from context, or you add something like “in the morning”.
In written form, 12h is rather uncommon, unless speaking colloquially again (e.g. texting with friends; not with businesses or clients, as that carries too much risk of ambiguousness and sounds too informal).
Can't say about Germans, French have these habits:
> 12h time can also carry a subtext or impreciseness
There's an additional twist where 12h time is expected to be precise: when being asked for time, many a time one would answer with 12h hour + smallest delta with ±2min error margin, and only sometimes 24h, possibly rounded:
"it's 10 to 3" (3h moins 10): could be 14:48 or 14:52, 14:53 would be "5 to 3" (3h moins 5). This has a slight bias towards the highest, as usually people ask so as not to be late somewhere, so 14:52 could be answered as either "5 to 3" or even "3" depending on urgency. Someone answering "3h52" happens at time but it's fairly rare and feels a bit odd; usually happens out of some habit when reading time from analog clocks.
It's also fair to say 14:45 when it's actually 14:41, but basically nobody would say "quarter to 14".
Also, "noon" is often understood as a quasi-synonym to "lunch", and could actually mean maybe up to 2PM for some, or even 11AM if there's some preliminary social or logistics expectation: "we'll come for noon" (on vient pour midi) has a different context-dependent subtext than "we'll arrive at noon".
> you add something like “in the morning”.
"In the mornin'" ("du mat'") is indeed used as a quasi-marker, a replacement for AM but only when ambiguity needs to be resolved, which it rarely does given known context about activities or people.
Often it's used for emphasis e.g earliness: "woke up at 4" has enough context to gather that it's in the morning but has a neutral, factual tone to it, whereas "woke up at 4 in the mornin'" evokes "what? shitty night I bet, you must feel terrible"; or lateness "we were out yesterday, went to bed at 4 in the mornin'!" similarly prompts "oh looks like you had some fun!".
Sometimes it is used in jest on 24h time. An answer to the above could be "15 in the morning? are you out of your mind?", implying that person would stay up late the night before or in self-derision about one's oversleeping habits.
It's vanishingly rare to literally say "in the afternoon", generally this is inferred or explicited from conversational context, as opposed to a marker: "let's see each other tomorrow afternoon, let's say around 3?"
When talking to someone, you’d say things like (literally translated, obviously) “let’s meet at five”, or “I saw them at around 5 in the afternoon”.
Using 24h time when speaking is common as well, but tends to emphasize formal time or preciseness. Something like “they are open until 18:30”, “the movie starts at 20:45”, or generally if there’s not enough context or a risk of getting it wrong.
So conversely 12h time can also carry a subtext or impreciseness, e.g. if I say “see you at five”, then often it’s around five, but if I say “see you at 17 o’clock”, I more likely mean 17:00 sharp.
Markers like “AM/PM” don’t exist, at all. When using 12h time, it’s either clear from context, or you add something like “in the morning”.
In written form, 12h is rather uncommon, unless speaking colloquially again (e.g. texting with friends; not with businesses or clients, as that carries too much risk of ambiguousness and sounds too informal).