Being from Louisiana, I'm so glad that "Where are y'all" is accepted. We southerners take pride in our "plural you."
I'm not even from the South, and I use y'all frequently. Seriously, I don't know why 'standard' English doesn't really have a plural you...
Fun fact: In English, "you" originally WAS the plural "you" and formal singular "you", whereas "thou" was the singular "you". Since "thou" is largely archaic, "ya'll" serves an important purpose, and I too use it all the time. :) correct me if I'm wrong.
I think ти is singular and informal (in other words, how you'd address a friend) and ви is singular and formal (how you'd address a teacher or a boss) or plural (how you'd address a crowd).
A lot of Slavic languages have this form as singular-formal form. In Slovak: Vy, in Russian, Ukrainian (and i guess also Serbian) ви. We used to use "Wy" in Polish, but it is archaic now. You can sometimes find it in films about military.