It will only get changed if we keep reporting it (with the report button - not here, which is just for discussions among the students). I hope you already did that.
I wrote "There are many gazelles in the Serengeti", clicked on Report and selected My answer should have been accepted.
Very frustrating, but if it is any comfort I have received 10 Duolingo feedback messages in the last 2 weeks that they have accepted other reports I made.
The Swahili course developers have surges of activity like this every so often, and eventually they will get round to fixing this question too.
UPDATE: I just got this reply from Duolingo (31 Dec 2018):
You suggested “In the Serengeti there are many gazelles” as a translation for “Serengeti kuna paa wengi” We now accept this translation.
And I have received 38 similar Duolingo replies in the 5 days since my previous comment. Things are looking up!