I know! I have been studying Latin for years, and I'm flying through these lessons. Do you also study Latin?
"Yeah" is not inherently rude, it's just a different register. Sometimes it's appropriate, sometimes it's not.
I don't know if Esperanto has different registers of the affirmative or if it only has "jes". If there is only "jes" then "yeah" ought to be a valid translation as well. In that case, you should flag (report) the lesson and select "My answer should have been accepted." The devs can't think of every answer to code in, so they rely on us to suggest things they forgot/overlooked.
But if Esperanto does have different registers the way natural languages do, then "yeah" would not be an appropriate translation of "jes".