Full disclosure, I have only tried a few language apps. Aka Duolingo and Mango. I have also done in person instruction for several. I am a terrible dabbler. But there are so many. I say having learned parts of six.
So, I've been learning Hawaiian on Duolingo since it was added in Beta, which at this point is about two years ago. I have learned a lot of words. I have googled a few things. Hawaiian is a verb subject object language (as opposed to subject verb object or all the other variations). I figured out some things.
I had previously used Duolingo to brush up on languages I had previously done in person instruction for. It works very well for that. Now the benefit of the immersion approach that Duolingo is essentially doing is that you figure things out for yourself. If a language has a word or character that essentially means this is possessive now, you will eventually figure that out if you are familiar with languages that do that. Similarly you might deduce on your own, okay, that word just appears to go in front of proper nouns. Also duolingo has an owl that cheers you on if that is a thing that you are into.
Mango stops to explain things to you. Not a lot of things so far. Mango also introduces you to words and then asks you to know them rather than giving you a phrase and expecting you to magically figure out what it means. (Yes, I know you can press on the word in Duolingo.) Mango then does review cards and asks you to self report whether you got it right or not. Which would be very easy to cheat on.
I am using the free version of Duolingo and the free version of Mango right now for Hawaiian. I am theoretically much farther along in Duolingo, but the difference in lesson progression means I am learning things in each that help me with the other.
The DC library card - and perhaps yours to - also has access to Mango subscription, Hawaiian just turns out to be one of the free languages so I haven't tested that part of it.