What kind of language should I learn to properly ask and answer here?
You can use any variety of English on SE sites.
Brits tend to use BrE, North Americans tend to use AmE, and people from elsewhere in the world use whatever English they know best.
In particular, on this site, English Language Learners, people use whatever bits of any variety of English they've picked up in their learning journey.
Does Stack Exchange have an accepted standard on language and spelling? Which is it?
For bodies, no. For tags, US-English.
The differences are so small that there's no chance of misunderstanding someone because they use one or another version of English, so you're free to use whatever you want to write content.
There may be significant differences when answering a question (e.g. a word, phrase or pronunciation may be very common in British English but almost unheard of in American English) but authors usually indicate so if they know about it. Even then, it's fine to use British English prose to talk about American English usage or vice versa.