Does koacja mean "the evening meal" or "the big meal of the day?" Maybe the distinction doesn't exist in Polish. Where I live "supper" is the evening meal, but "dinner" is the big meal of the day. For instance, on Sundays "dinner" is often midday. During the week, the big meal is in the evening when everyone gets home from work and school.
"kolacja" is the evening meal. Well, it can be fancy, it can be perhaps the biggest meal of the day on some special occasions (Christmas Eve), but generally obiad is the biggest one.
"Dinner" is accepted. It is worrying that more and more people say that an answer is not accepted, although in the Incubator, it is...