Why is it not "Are you dirty?"?
I think you'd need a topic for that, meaning you'd need to put あなたは in front of the already existing sentence.
You never need to state the subject or topic, if it's clear from context. So, this should be just as possible to translate as "Am I dirty?" "Are you dirty?" "Is he dirty?" "Is she dirty?" "Are we dirty?" "Are they dirty?"
"Kitanai desu ka" Shouldn't it be "isn't it dirty"
きたない is not a negative. The Kanji shows that the 「ない」 is part of the word itself, not a conjugation:「汚い」.
I think I hear "stanai" and not what i expected, "kitanai". Is "stanai" the correct pronounciation?
No, it's not. I don't hear "stanai", I hear "ktanai"- with the "i" devoiced.