You need to keep in mind that things are expressed differently in different languages. ' Habari ya/za mchana kaka' does not really say 'How is (your) afternoon, brother? ' - that sentence is about the closest english way we can come up with... Literally it translates 'News of the afternoon, brother ?' with 'ya' asking for one news / with 'za' asking for several news.
This should be translated as, "How is your afternoon, brother?"
Not "good afternoon brother." Those are two entirely different things in English, and the solution was to translate it to English, lol. Not to a kiswahili person learning English. That's a question -- in English it would be a question. This feels intentionally confusing and misleading, and I think that reduces morale. Just when I think "Oh I get it," I get it wrong because of some arbitrary Duolingo quirk
Thats a good point. My understanding is that the literal translation is "News of the afternoon, brother?" But it is used as "good afternoon". Of course, some might want to actually know and others are just greeting each other. In that you would have to ask a native speaker for the nuances of ya vs za for a question.