I had this sentence as "Write this in Finnish" and it rejected "The hardworking Russian speaks Korean.", demanding "Russian person" instead.
No Finn I know would say "Russian person" or "Russian language" instead of just "Russian", but I guess the exercises want to underline that nationalities, languages, and countries all have a different Finnish word (e.g. venäläinen vs. venäjä vs. Venäjä). All natives know the difference (except many capitalize languages, thanks to English), though, which is why it feels strange to me to be required to add the extra noun.
Kyllä, tarkoitan, että maat ja kansallisuudet kirjoitetaan isolla, ja kielet pienellä.
suomalainen - Fin (mies), Finka (nainen)
suomi - (język) fiński
Suomi - Finlandia
Puolaksi voi sanoa fińszczyzna/polszczyzna/czeszczyzna, mutta yleensä sanomme (język) fiński/polski/czeski.
You are very kind. Mind you, be careful of applying too much meaning to large numbers. I got many of the level 25s when Immersion was around and getting to that level meant something, especially in the reverse trees. Today, getting to 25 does not mean that much. For example, speed testing few skills ad nauseam to rake in XPs does become boring. I think there are people who are past level 25 and they have only finished few skills of the tree. The stories, when available, are valuable. I understand some people speed test through them as well to get brownie points. Also, my main interests involve translation work which makes it a little easier to move between languages as I don't have to spend so much time on listening/speaking skills. Unfortunately, any real work in this area is no longer part of Duolingo, so the incentives to stick around have diminished. Keep owling as long as you LEARN something!