"Have a good afternoon Esther"
Translation:Mchana mwema Esther
"mchana" is afternoon and "siku" is day. I don't see why it's wrong to exclude "siku" as a possibility for translating "afternoon".
In the mulpiple choice question, you should now give both answers as a correct translation, so possibly you are correct. Saying Good afternoon seems to me a bit unnatural in English, though it is very common in Dutch. The Dutch word voor good afernoon is "goede middag" which is shorter to say I guess.
As a greeting, "good afternoon" is not unnatural. But "Mchana mwema" is not really a greeting. As in you don't lead off with it in a conversation. It's more for at the end of a conversation when you would say like "ok, well have a good afternoon" in which case saying "good afternoon" (by itself) would be unnatural.
I agree, we say it frequently. By the way, the Spanish is "buenas tardes", because tardes is feminine. You have an impressive array of languages in your comment! :)
Good afternoon sounds perfectly natural to me as a native English speaker.
There is nothing whatsoever about saying good afternoon in English, good day is rarer and is often associated with Australians.