For this one, it's a bit hard to explain. In english, both a topic marker and a plural marker often adds an S to the end of a word.
So if you want to buy multiple of something, or if you simply like the subject of it, both english sentences come out to "I want five mangoes" and "I like mangoes."
In ‘Ōlelo Hawai‘i, however, the plural marker, nā, is just for literal plural things. If we are talking about the subject or preference of mangoes then we don't use "nā." It may be better explained with these expanded examples:
"Makemake au i ka manakō" I like mango--as a subject, topic, or preference.
"Makemake au i nā manako" I like multiple mangoes. I want multiple mangoes.
The second one is moreso saying, rather than liking mango as a fruit in general, that I am liking a literal group of mangoes. So maybe if I'm evaluating the quality of a crop, and I like how all the bananas in the crop look, i could tell the farmer "makemake au i nā mai‘a."
Just have to remember to reserve nā for "literal plurals" only. :)
Aloha! I am a native speaker and I do agree, though it should moreso be "Hawaii's people" or Hawaii people" as a direct translation. They should add these as options and even allow yours maybe with a note on how to simplify it.
I have omitted the word "of" from my direct translations because we actually have a separate structure to specifically say "of something." So rather than people of japan, po‘e Iapana is just Japan people or Japan's people.
Side note: in some contexts, po‘e Hawai‘i may indeed be translated to "Hawaiians" but quite a few of us prefer not to use that way as we have specific words for the native Hawaiian people. Po‘e Hawai‘i should more often be used to generalize Hawaii's people whether native or not, like a residency term--at least that's the way the younger generation seems to prefer it.
Just know this is one of those things that will end up depending on context in the real world.