I'm actually from the USA. Texas, at the moment, unfortunately.
I've never seen "sth" before outside of these forums. I'm quite unfond of abbreviations in general, partially because they make it very difficult for communication with people who either don't speak English natively, or have to depend on translation software or reader software (visually impaired) to access our forums.
It's generally best to avoid all, or almost all, abbreviations here so that they don't provide a hindrance to those other users. Generally it would be best if users attempted to speak the closest thing they could to proper English, when possible. I'm not going to get on you for say, spelling errors or grammatical missteps, but things done intentionally that are capable of obfuscating your meaning can provide problems for other users very easily.