Sth is a specialised abbreviation for something. It is only used in cases like phrase books and other reference books where the premium on space makes considerably heavy abbreviation appropriate.
Compare how a dictionary might use abbreviations like adj. phr., esp. Brit., colloq. and obs. but in normal English we would never use those but the full phrases. It makes sense though because:
- Space is at a particular premium in dictionaries, which struggle to be both comprehensive and physically compact.
- The context means that at the point where we see e.g. adj. phr. we are expecting a part of speech, where we see colloq. we are expecting a note on usage and so on, so the problem with such heavy abbreviations causing confusion is reduced.
- The dictionary will contain a key to the abbreviations it itself uses, further reducing the problem with heavy abbreviations.
Similarly, sth (and sb for somebody) are found in phrase books and vocabularies for non-native speakers. Whether by force of habit, or by confusing the abbreviation listed as for use in that particular context as an abbreviation defined as one used in English, some non-native speakers are led to believe that it is a common abbreviation in English (akin to etc. or i.e.) and so use it, though native speakers do not generally use these abbreviations at all and so find their use jarring at best, and at worse so opaque as to not understand what is meant.
It's an interesting foreignism, because it results not from lack of fluency, the influence of another language, or a hypercorrection in attempt to avoid those (which cause most traits that one will find in non-native use but not in native), but from an artefact of the very guides that are ironically intended to help people learn and use the language.