I think "y'all" should be acceptable in addition to "you" for σας since it's the closest English equivalent. I know that isn't a proper English term but it's what makes the most sense to me in distinguishing between σας and σου. There is no plural form of "you" in modern English other than the informal "y'all". It used to be "ye" in middle English. Let's adopt "y'all" Duolingo!
Indication of the used pronoun: