why is it not venitul anual tau? Is it because it is the genitive form and if so, would my translation also be correct?
Im not sure if it is because of the genitive form, but i can assure you that the correct translation you wanted to go for is "venitul tău anual". But you were close enough! :))
You could say "care este venitul anual al tau?" and it would be correct.
Think of it like "what is (care este) the income that belongs to you (îți)", and then change a bit the order (care îți este / care-ți este )
In most european countries it is very unusual and even inappropriate to talk about money and income. Is this an appropriate question in Romania?
No, it isn't. But people with very low income are discussing it frequently. Everybody would, still, use "salariu" instead of "venit".