How can I make Google Now talk back in other languages than English?
Firstly, my best wishes for your brother. 

As for your question. My phone is in English. But I set the Google Now to understand both English and Turkish. Please note, Google Now itself is in English. But it understands Turkish commands too. I believe the very same can be done with Danish as well.

At Google Now, go to:


Settings
Voice
Languages


From there, choose Danish.

In my phone it looks like this: Default Primary: English(US) Others: Türkçe(Türkiye)

P.S: Google Now decides what language it should be based on your phones language settings. So in order to make the app in English, your phone too must be in English. Since your brother has no problems with that, I don't think that'd be a problem.