"Y'all" is plural, "all y'all" is more plural.
For instance, suppose you know a family of people. Two people out of that family are currently visiting you. You might say "Y'all should bring the whole family over next time; all y'all are welcome anytime." The first "y'all" refers to the two people visiting; the "all y'all" refers to the whole family.
What about "Vou" from the French "Vous" ?
You can use 'thou' to force Google translate to translate from English to the informal form of another language. Otherwise, it will give you the formal translation. This is safer if you don't know the social/cultural implications of using the informal form (it can be insulting or inappropriate in some contexts), but it's not always what you want.