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.
My brother's books are in the green suitcase.
De boeken van mijn broer zitten in de groene koffer
( The best teacher is my last mistake )