It's a bit misleading to use "Go Hilla!" as the translation and it might cause confusion later down the line. Bravo Hilla! or Well done Hilla! might cause less confusion in the long run and help people learn the actual meaning of the word Hyvä. I'm under the impression that "Go Hilla!" is a highly informal: local way of saying things and it's easy to misunderstand it in text without context since there are two ways it can be interpreted.
"Go, Hilla" sounds good to a speaker of American, and I expect Canadian, English as a way to encourage someone. Note, the comma is standard in a vocative expression. Otherwise it sounds as though you mean "Hilla" as a destination or manner of going, as in "go crazy", "go south", or "go sour".
I would be surprised if a speaker of British English found the expression confusing. Though since many users of Duolingo speak English as a second language, perhaps it is fair to say this might confuse Duolingo users.
This is meant to be a constructive cheer, for example, to encourage a runner down a race track, so it will always be idiomatic and contextual, that is simply how an encouraging cheer works. The alternatives being offered are congratulatory in nature and, in fact, have a different meaning. If you have ever been to a sporting event or competition you know it is common to hear, “Let’s go (team/player name)!” I understand the confusion, but it does not mean it is incorrect. Perhaps (idiom) or (cheer) could be appended for clarification, but we should be able to use a little intuition and interpretation in translations that are not literal.
I'm a native speaker of Finnish. I tried "Way to go Hilla!" which in the sense of praising someone's achievement is just a longer version of "Go Hilla!". Though colloquially "way to go" might have some sarcastic undertones in English, I think it should be accepted.
Ok, I am not a native English speaker too. But it is a common expression in Brazil ("Boa, Hilla!"). We don't use it to cheer a team (like we were in a croud), but we can use to cheer a teammate (or the team you are playing on) when he scores or make a good move, for example.