Translation:– Hello! Who are you? – I am Väinö.
I agree in English it sounds impolite, but it's also a manner of translation. It may translate to English in a way that English speakers consider rude, but not hold that same connotation in Finnish. From my experience, Duolingo usually teaches the more "Formal" greetings etc first. Hence why it uses Hello, instead of Hi. "Terve, Hei"
I guess it's because "ä" and "ö" are not variants of "a" and "o" (the dots are not accent marks) but vowels/letters of their own and it's really important to pronounce them right and mark the difference or you'll easily end up saying the wrong word. If we take the name "Väinö", for instance, and pronounce or write it as "vaino" that'll mean "persecution in Finnish. :/
If Duolingo accepted "mä olen Väinö", it should accept other spoken variants as well. And since alternative translations are added one by one by the volunteer team, that's just a bit too much to ask right now. This is just a course on mainly written Finnish or kirjakieli for now, but of course that might change at some point too. :)