Translation:Isn't the baggage heavy?
The answer is asking "is it heavy?" in a polite way by saying "isn't that heavy???"
By asking in a roundabout manner, the person speaking is trying to figure out if the person holding the heavy bag needs help carrying it. "Isn't that heavy? Here, let me help you..."
The way you're phrasing it, makes the bag sound light, which is the opposite intention for the question.
"Is that not heavy?" I would understand as the same as "isn't that heavy?", the first one is kind of old fashioned phrasing but both work as a kind of "don't you want help?" in English, to me (I'm a native speaker from Ireland, perhaps it's different in other Anglophone regions)
Generally, Japanese doesn't specify singular or plural since you can usually tell from context. I would be hesitant to accept luggages (if that's what you wrote) though, since I feel (and Wiktionary agrees with me) that "luggage" is generally an uncountable noun and thus takes the singular no matter how much luggage there is.