Why is 'luggage' not accepted as correct translation for 'koffer' ?
It's something different: 'luggage' means bagage.
Shouldn't "bag" be an accepted translation to "koffer"?
Koffer is a word used for a specific type of luggage. In my humble opinion, a suitcase is not a type of bag, so no. Depending on the type of bag, "bag" would be "tas" or "zak" in Dutch.