When I was in college, I took a New Testament Greek course. The professor hated that I translated second person plurals as "y'all," but when I explained to him that I wanted to differentiate the singular and plural in my translation, he begrudgingly accepted me doing it for that quarter. The next quarter, the continuation of the course was taught by a professor from the South (I'm in the US, by the way). He loved the fact that I differentiated, and preferred his students do so.
If traveling in America, using "y'all" might get you some raised eyebrows, depending on where you are. Generally, in most places around this country, you can get by quite nicely by either expanding the contraction back into "you all," or using the phrase "you guys." Some places have their own distinct dialects, but these should serve well no matter where you go.
One option that I use when the people I'm addressing are of a similar age to my parents is "folks." That one is good for a group of either all men, or mixed company. If it's all women, then I tend to default to "ladies."
Moral of the story: know your audience.
"you" was originally the plural form (you all) and only used as polite form to address a single person (I suppose it came from the French); at one point in time the distinction between "normal you" (thou) and "polite you" got lost and only the polite you remained. If I'm not mistaken, Russian is currently undergoing the same change: they still have the singular form, but they seldom use it. Even friends and people of the same age group tend to use the "you all" form.
Russian "underwent this change" (the you plural becoming the you polite) a few hundred years ago, they likely got if from the Germans (blame Peter the First). The previously singular form is only used in familiar circumstances, just like the German Du. And yes, close friends who emphasize respect for each other, call each other Вы (the respectful/plural form), but it has nothing to do with the singular disappearing.