It's not a problem of Polish, but of English. Simplifying, our śniadanie/obiad/kolacja are breakfast/lunch/dinner in the US, and breakfast/dinner/supper in Great Britain. It seems that even in Great Britain it isn't that popular. But we in Poland are usually taught the 'more British' version, that's why for you (and me as well) 'obiad' is rather 'dinner'. But Duolingo is an American company, so we usually go with the American translation as the default.
Well, Duo uses American English, and American English calls the three main meals (śniadanie/obiad/kolacja) breakfast/lunch/dinner. I don't personally like this option, most Polish people I know were taught the British version (breakfast/dinner/supper), but this is what we had decided upon. We used to star both versions (lunch/dinner), but people were confused a lot. Since we went with 'lunch', I believe the only people that complain in the comments are Polish users.
"drugie śniadanie" actually works for "lunch".
The British English version is breakfast LUNCH and dinner/supper. Dinner as a main midday meal is fraught with social class problems, Dinner in the evening is formal, supper is a normal evening meal. The derivation of the word lunch comes, I think, from an old word meaning a hunk of bread eaten somewhere between breakfast and dinner.
I do, but this is exactly what we decided to do.
Before we decided on such translations, there was so much complaining about obiad and kolacja. Seriously, the current level of complaining is like 5-10% of what we used to have. So even if I myself would translate "obiad" to "dinner" and "kolacja" to "supper", we're going with the American translations.