As well, the South African “howzit”, which from my observation isn’t even actually a question but just invites the other person to say what they wanted to say.

I've always considered the standard reply to be "yeah nah, not too bad, yourself?". I think that it's possibly more Kiwi than Australian though.

In Germany, people say "Na?" or "Na du?" to which the other person usually responds the same.

It's not easy to translate but I think it's quite similar to the greetings mentioned above. Literally it's something like: Well? You? (and not 'well' in the sense of goodness, but rather questioning)

It's an interesting greeting and as a foreigner seemed very odd at first.

We also have this in certain corners of the U.S.:

A: "'sup [man]?" B: "'sup?"

(It's also stated only has like a half-question or less—the usual question lilt is deemphasized.)