In this sentence you could use either. Niemand is a bit more formal and can generally only be used for persons, though.
– DonHolgoNov 21 '18 at 20:31
@DonHolgo: so you have to use keiner if you want to be clear that the dog doesn't speak German either?
– TonyKNov 22 '18 at 13:54
@TonyK: I guess so, although probably nobody would understand the sentence with niemand to say that maybe the dog does. I rather wanted to point out that it's wrong to use niemand when talking about balloons and saying keiner ist rot, for example.
– DonHolgoNov 22 '18 at 22:37
keinerif you want to be clear that the dog doesn't speak German either? – TonyK Nov 22 '18 at 13:54