Same here in China. I grew up learning English using mostly British textbooks and sth is introduced to us quite early, pretty much like xyz in algebra. I'm actually quite surprised to find out it's perhaps not that common in the US.
In my experience you'd be hard pressed to find 'sth' used in the US (same for sb), most people would type it out to avoid ambiguity with 'south' and other words. In SMS and other short formats, phonetic spellings like 'sumthin' are more common.
I've only ever seen South abbreviated as S, never as sth.
