"I am interested in helping other people."
Translation:Interesuję się pomaganiem innym ludziom.
"Interesuje mnie pomaganie innym ludziom." would be also correct, but both sound unnatural.
The thing that you are interested in (helping) is in Instrumental. "pomagać" takes Dative, so "other people" is in Dative.
Damn, I must be really tired today. You are right of course, it's Dative.
Edited my comment, thanks.
Yes, but it sounds as if you came to a charity organization and say that you want to be a volunteer there. Which in a way makes more sense then putting it like "My hobby is helping other people". And actually it already works.
But imagine "I am interested in tennis". "Interesuję się tenisem" is a perfectly fine sentence. "Jestem zainteresowany tenisem" sounds rather weird. It's as if you were going to buy tennis. Or perhaps tennis is one of the options that someone offers you and you are interested in hearing more about it. So there may be some contexts when it makes sense, but they are limited.
Translating this sentence sucks. If you google the whole sentences in Polish enclosed in "" on google.pl, the only link that comes up is this thread on duolingo. I've never heard anyone say anything like that. Poles don't seem to think, write and talk like that. You either help people or not. You can be interested in i.e. astrophysics, not in helping people. Google "Interesuje się astrofizyką". I get 463 hits instead of 1. And the original translation sounds like a sentence taken from a first draft written by a somewhat dim-witted 12-year-old.