"No, thank you."
Translation:Nee, dank je.
It can be either. Because the english you doesn't make it clear if this is a formal situation or not. They say "you" regardless.
U is formal, so older people superiors usually strangers (but if you are the same age and it is clear you are both casual sometimes je is ok. Like to the person standing next to you at a concert. But you will use u to ask a stranger directions. Though often people take the middle road. Can I ask something, Mag ik wat vragen? Instead of having to choose, mag ik je/u wat vragen. In case someone might be offended)