"What do the whales eat?"
Translation:Hvad spiser hvalerne?
I'm not a native Danish speaker and I certainly do not know the language well enough to answer your question. But I do believe that vagueness and ambiguity are part of all languages. So I wouldn't be entirely surprised if the answer to your question was "Hvad spiser hvalerne?"
Of course, as with any potential ambiguous utterance, you may, as a listener, have to ask for clarification. Or as a speaker you may clarify what you are saying by giving more information to the listener (e.g. "Hvad dyr spiser hvalerne?" ("What animals are eating the whales?"