"Why are you learning Swahili?"
Translation:Kwa nini unajifunza kiswahili?
Mbona indicates surprise. Like "How come you're learning Swahili??" or "Why on Earth are you learning Swahili?"
Kwa nini is neutral and is simply asking for the reason, not suggesting that you shouldn't be learning Swahili.
'unafundisha' sounds like 'you are teaching'. Towards another person and Not 'you'.