Let's start with Hungarian, to get us going. Now in Hungarian there are many ways of greeting someone, many of which are entirely dependent on a perceived or actual relationship between greeter and greetee.
Informally, you can go with szia or szervusz, unless of course you are greeting more than one person, in which case it is sziasztok or szervusztok. Apparently szia is slightly more informal than szervusz, but this is not something I've ever really encountered. You can also, as I may have mentioned before, actually use hello.
However, even if you know the person, you have to be a little bit careful with such levels of informality, because saying it someone to whom you ought to be granting some form of respect (older people, VIPs, etc), could cause offense. Even if you don't reckon they've earned that respect. To an older woman, for example, you are supposed to go with Csókolom which is sufficiently respectful for that group. (However, I occasionally worry that if I offer a Csókolom to someone who is younger than me, or around the same age, or just marginally older, but who seems "obviously" older, I risk causing age related offence).
To an older man, or someone you don't really know (assuming they don't fall into the "older woman" category), you need to offer some form of good morning/day/evening etc. This would be jó reggelt (good morning) or, gussied up a bit, jó reggelt kívánok (I wish you good morning - it's not clear what else you might be doing with your "good morning" if you're not wishing it, but there you go). Others include jó napot (good day) and jó estét (good evening). OK, so far so good, but then you have to remember that what you might consider the morning is not necessarily the morning to a Hungarian (nor to a Romanian for that matter, but we'll come to that). Say jó reggelt at 11am, and you are looked at like you are mad. In villages it's even worse, because the morning very definitely seems to finish at 9am. A 9.15am jó reggelt would probably get you kicked out of the community for being a lazy good-for-nothing who didn't actually wake up at 4 to feed the chickens (or whatever it is that people do at 4am)
It's been 6 years now, and I still mix some of this stuff up. Yesterday I was going to pay the phone bill and saw someone I sort-of-half-knew, and offered a cheery szia. The moment it left my mouth I knew that this was almost certainly a jó napot situation and that I had erred. The really tricky thing is that if you are greeted first you don't necessarily respond in kind, like you do in English. Children (especially polite ones) tend to Csókolom all adults, and I may have inadvertently confused one or two kids in my early days here by offering the same greeting back.
OK, so that sums up the greeting challenges in Hungarian, but here of course there is another level of difficulty - the fact that some people you meet aren't actually Hungarian, but Romanian (this being Romania and all that). Thankfully, Romanian doesn't seem quite as complex as Hungarian in this regard. Here in Transylvania there is the use of szervusz (though I am sure it's not spelled like that by Romanians), but apparently that's only here, and not elsewhere in the country. Then there are the various good (insert period of day) greetings, in which like Hungarian, there is a "different" understanding of what constitutes "morning". I said bună dimineaţa (good morning) to two Romanians I met in England a couple of weeks ago (at about 11am) and they laughed as if I was an idiot, and said "Dimineaţa?" with heavy emphasis. You can just about get away with the bună bit on its own most of the time (you couldn't just say jó any more than you could just say "good" in English).
The extra level of challenge here is knowing who you should greet in Hungarian and who in Romanian. Obviously if you know them well, it's no issue, but sometimes you sort of half know someone, but can't actually remember what their first language is. There are two Romanian blokes who live in my building for example, but I am always confusing them with a Hungarian bloke who also lives there, and so I frequently guess wrong (and I never seem to learn).
The only one I always get right is my own slightly pathetic little rebellion against organised religion, whereby I always offer a cheery jó napot kívánok to any Orthodox priest I walk past on the street (knowing full well that he must be a Romanian), and a similarly breezy bună ziua to Catholic priests (knowing full well that they must be Hungarians). I like to think it throws them a little, and it makes me feel vaguely smug for a nanosecond, so that's a result in my mind.