"I fell down"
I'm a native speaker (although my Swahili is getting rusty having moved to an English speaking country, which is why I'm on duo lingo). I fell in English can also be implied that you fell down and it would still be correct if you didn't use the word down same as Nilianguka in Swahili can be implied that way. Although the correct translation of the sentence given would be I fell down as they've specified it in the sentence. Hope that makes sense.