Hi user, answering from the UK so depending on where you have moved the answers may be slightly different, but in Yorkshire:
>no worries
an appropriate reply to, and how does it differ from
>no problem?
both mean the same thing, I would say 'no worries' is slightly more informal, so if your friend thanks you for doing something, reply no worries. If you boss asks you do do something, or thanks you for doing something reply 'you're welcome' or 'no problem, anytime' . Generally though both are just as acceptable each other.
what do i reply when i walk into a shop and the assistant says
>you're right there?
Either say 'fine thanks, just browsing' or if you want a certain item reply ' have you got X?' or 'do you sell X?'
is being addressed
>mate
friendly or aggressive?
Generally it's a term used when one guy is addressing you and he doesn't know you. For example, if you held a door for me and I had never met you I'd say 'thanks mate'. If you bump into someone in a bar or club, say 'sorry mate'. It's a firiendly none confrontation term for another male.
how do i order a flat white?
>can i have a ...
or
>may i have a ...?
Both are fine, I'd personally go with the first.
what is the difference between
>peng
and
>neng,
or do they mean the same thing?
Never heard of peng or neng, sorry
how does a 20-something say goodbye to a 20-something?