Sometimes I encounter ain't, but I really don't know how to translate it properly.
What does ain't stand for? If I really wanted to use it, in which contexts would you say it's acceptable using it?
Ain’t is a negative present-tense form of the verbs be and have employed in all persons and numbers:
It represents a coalescence of the ordinary spoken contractions of not and the three relevant forms of the two verbs:
It is used wherever be not is used: as a copula, in progressive constructions, and in passives; and where have not is used as an auxiliary, in perfect constructions.
Ain’t is not slang (which means, roughly, a fairly novel usage employed by an ‘in-group’ as a token of their ‘in-ness’) but a colloquialism which was at one time used virtually universally. You find it very often in 18th- and 19th-century plays and novels, in the mouths of persons of high social standing.
However, it aroused particular hostility among 19th century schoolmarms, who assaulted it ruthlessly and succeeded in painting it as the mark of illiterate speech. Accordingly, it should not be used in formal contexts except as an ironic nod to the vernacular. It is still very common in speech, but regarded as sub-standard rather than merely non-standard.
—Jon Purdy (link)
The definition here says it all.
So, basically, it's slang, but there are times when no other word fits. I wouldn't use it in a formal situation, except to quote a title. I'd use it sparingly except in very informal situations.