"Whose glass of juice is this?"
I guess it's correct from the view point of a Chinese... The 'correct answer' is a little bit better though
it sounds like you are asking about the juice from the glass not about the owner of the glass, so i think is wrong, at least the meaning for sure.
However, the ownership is specified. In terms of Pifagor's wording, he can mean "Whose juice is this (glass)?"
To make the alternate translation more simple, we can say 這是誰的果汁？
Google translate for "whose glass of juice is this?" is 这是谁的果汁 Why is the DL translation for the same different? which one is right and which one is wrong?
They are both right. The DL translation is asking 'who's cup of juice is this?' and the google translation is asking 'who's juice is this?'. The word '杯' meaning glass of or cup