Could this sentence be used to offer coffee after lunch? Because we sometimes say it like that in spanish.
Not nesessarily after lunch but yes, it can be used to offer coffee - especially if there are many people.
How would one say, "Who will have drunk coffee?" I thought it was also this one
so this can be "who will drink coffee" and "who will be drinking coffee"? Is not the same, is it?
I believe that despite the fact that at first sight 'who will drink' sounds perfective, it doesn't have to be. Both seem to be correct translations. "Who will be drinking" is better, as it shows the 'continuity' of drinking.