Well, obviously I can only tell you about Germany and France but I (and that's not just me it's the custom) would always use the formal form. Always. It's rather rude not to do it, even if the other person doesn't mind. I'm not even in my twenties yet but older people tend to use the polite form when talking to me. So yeah, it's never wrong to use it and people will offer you to use the informal form, of that I am sure :)

Oh, OK. So it's like Russian - which I have a slightly rusty intermediate knowledge of. I guess I knew about შენ and თქვენ but hadn't connected them yet with actual language. Also, I wonder if my age will make a difference. If I were in my twenties and were meeting a new person my own age in a social situation, I'd use the informal form right away -- but being older, if I'm introduced to someone around my own age, would I be expected to use the formal form at first? Or would I only use that if the person has some special official status? I'll be interested to find out some of these customs.

oh dear, that looks very messy, sry about that. but I think you got that ხარ is singular, -თ is always the sign for plural ;) Anyways, I would like to ask a Georgian speaker why people sometimes say როგორა instead of როგორ?

შენ ხარ means "you are" in the second person singular. თქვენ ხართ means "you are" in the second person plural. It is also the polite form (when talking to a teacher for example). English doesn't have that difference but many foreign languages do like French, German, Spanish... dunno what other languages you know so you use როგორ ხარ if you're talking to only one person and you know that person well. use როგორ(ა) ხართ when talking to either more than one person or talking to someone you're not familiar with