The difference is grammar. Polish has declension of nouns. człowiek is Nominative, człowiekiem is instrumental. Polish has two ways of translating He is human.
On jest człowiekiem (instrumental)
On to człowiek.(nominative)
Only one of those can be also applied to "me, you, we, you" "jestem człowiekiem"
Read more about "to" and instrumental at: https://www.duolingo.com/skill/pl/Defining
So how would you say "I am a person"? Is there a more relevant word?
"Я человек" in Russian is "I am a person", so why is the cognate "człowiekiem" not able to be translated as "person" as "человек" can be?
I am a little confused about the English translation. What is the difference between "I am a human" and "I am a person"? Why is the former right but the latter wrong? I mean, obviously there are two English words, so I know there can be a difference, I just don't get what it is.
Your question is justified (do not worry about the downvoting... )
The sentence "Ja jestem człowiekiem" refers to the most general definition of a man, human being
(człowiek, istota ludzka) as one
of the members of the species homo sapiens. In that sense it does not refer to the gender or character, so it has little to do with the words like "a male", "a person", or "an individual" which might be
true, but they have much narrower, at times abstract meaning.