"I am ashamed of you."
Translation:Wstyd mi za was.
"You (my children) did something awful and I am ashamed because of that and because I am your father and I failed in your upbringing" - I'm not sure if that's the only interpretation that English allows, but it's the one in Polish.
"za was"/"za ciebie"... well, I don't know how to explain it, but that's just what Polish uses in such context. I guess it's also similar in "Zrób to za mnie" (Do it instead of me, while I'm gonna lie on the sofa and watch you work), and probably some other constructions that do not come to my mind now.
"Wstyd" is the noun form of "shame", I guess "wstyd mi" here is more of a fixed phrase. Using a verb: "Wstydzę się" would work perfectly as well (no 'mi', then - it's like 'shame to me' vs normal 'I'm ashamed').
We discussed the exact meaning of "Wstydzę się za ciebie" and "Wstydzę się ciebie", then consulted those meanings with our British contributors.
"Wstydzę się za ciebie" is "I'm ashamed of you", "I'm ashamed by what you did."
"Wstydzę się ciebie" is "I'm embarrassed by you", "I don't feel comfortable changing my clothes in front of you", "I feel embarrassed being seen in public with you.", "You make me feel uncomfortable.". Those are the interpretations we came with.
It seems that they wouldn't be put as "I am ashamed of you" in English.