Translation:I need a Dutch-language dictionary.
http://www.lexilogos.com/english/dutch_dictionary.htm I'm too lazy to pick out a good one right now, but that has a few options.
I don't know any native speaker who would say "X-language dictionary" in English. It would be "I need a Dutch-English dictionary," or "I need a Dutch dictionary," depending on whether they needed the translation into English or just the meaning in Dutch.
Is Duo's preferred answer a regional usage, or just an awkward literal translation?
I just tried something on a whim... I knew I could have just used "need"... but I don't use the float over hints at all any more... I wonder why they do NOT accept: "I have need of a Dutch dictionary" which is essentially pretty much what they ARE saying? I suppose it's just a thing about their language, but I would think they could still accept that since it says the SAME thing.