German has auschlafen, which means "sleep out" and is similar to "sleep in" (the US version) except it means to sleep until you wake up naturally. These can cause confusion because I've known Germans and Americans to assume that the expressions mean the same thing as each other, since there is overlap.
If you go to bed at midnight and sleep till noon, you could be sleeping in (US) without sleeping out (German), if you set your alarm to wake you up, instead of waiting for it to happen naturally.
Same in Dutch. One thing that is true about the Dutch phrase though (and maybe also about the German, but I just don't know): if you wake up at more or less the same time as you normally do, you have not "slept out" (ausgeschlafen/uitgeslapen), even if you did wake up naturally. Uitslapen requires both waking up naturally and waking up later than usual.
(When waking up naturally but early, you may be+ slept out (= fully awake, having gotten all the sleep 'out of you'), but that's a separate issue, and you +have not slept out.)
This New Zealander would use "sleep in" to mean both oversleeping and spending longer in bed than usual on a day off for example.
A. Sorry I'm late! I slept in.
B. I slept in this morning for the first time in ages- it felt great.
#19 You are right. I know you like to hear this. To my defence: there are a few references to be found on my internet with 'sleep in'.
Sleep in, for me, implies intent.
For example: The blizzard was raging in the evening and school was cancelled for the next morning, so all the kids could sleep in.
or: I stayed up late doing XYZ because I knew I could sleep in the next morning.
Sleep late or oversleep is for situations where one was supposed to wake up at a certain time and did not.
"to sleep in" in Australia and UK for accidentally oversleep, but not exclusively - it could also mean deliberately.
"lie in" for staying in bed after you wake, also a "long lie" in Scotland
To be honest, I thought "slept in" was fairly universal for accidental oversleeping, even in the US. Maybe it was regional there.