"You all" or "y'all" is an idiom and is not proper usage. Plus it is not even used widely throughout the English-speaking world. Only in one part of one English-speaking country.
If you go down that route, then they would have to allow "you guys" (US/Intl. usage); "you lot" (UK usage); "youse 'uns" (Belfast usage) etc.
All ways to distinguish you (sing.) from you (plural). But all are idioms and local usage.
Believe it or not, once you've lived in the South for a while, you can develop a real appreciation for the term, " y'all." While it has none of the formality of vous or ustedes, it is very handy to have an unequivocal second person plural. The term is widely accepted in the region, so to say that is not proper usage is debatable. Further, English is by nature an extremely idiomatic language. I would agree however, that it is informal usage and should be avoided in writing.
Only feminine and some other a-ending nouns have unique accusative forms in Polish (and only in singular), all other nouns have either accusative=nominative or accusative=genitive.
masculine personal (boy)
singular : N-chłopiec- G chłopca -A chłopca
plural: N chłopcy G-chłopców A chłopców
Masculine animated not personal (dog)
singular : N-pies G-psa A psa
plural N psy G psów A psy
Masculine not animated (pencil)
singular : N ołówek G ołówka A ołówek
plural N ołówki G ołówków A ołówki
singular : N dziewczynka G dziewczynki A dziewczynkę
plural N dziewczynki G dziewczynek A dziewczynki
singular : N książka G ksiązki A książkę
plural N książki G książek A książki
singular : N okno G okna A okno
plural N okna G okien A okno