Ughh... "Hakuna matata" is a phrase coined by Disney's The Lion King. If you go to any Swahili speaking country and say "Hakuna matata!" to a native, you'd might as well pin a sign on your back that reads, "I'm a tourist, please rob me."
If you're looking for a way to make a native smile, say "Hamna shida." That's the "hip" way.