Im polish. I dont know any person who would say "lunch" as "dinner". In polish language long time wasnt word "lunch" it comes from West. "Lunch" is more like "drugie śniadanie" (second breakfast). And "obiad" you can eat only by yourself in home, not always with family or in restaurant. "Dinner" is a better option for "obiad" than "lunch", I promise ✌
This depends on whether we are translating into English or into Polish. English 'Lunch' is a meal taken in the middle of the day, NEVER in the evening. Dinner can be taken at midday or in the evening. However, be careful with the use of 'dinner'. It is a word fraught with social and class nuances. For Duolingo Polish purposes, I would always translate obiad as lunch.
Obiad is the main meal of the day, usually eaten around midday (12PM to 4PM). It is usually translated as lunch (because of the time of the day when it is eaten), sometimes as dinner (since it is the main meal which is often eaten socially – with family members, in a restaurant etc.)
Kolacja is a medium-sized evening meal, usually eaten between 6PM and 9PM. Again, since the conventions for naming a meal of this kind in English vary, it can be translated in two ways: as dinner or supper.
Eating habits vary between countries, and a simple equation doesn't work here. Even we British can't agree when we have dinner. For me, at the time "obiad" is eaten, I have (perhaps a late) "lunch". But although there are many in Britain who would call this "dinner", the only time I have dinner in the middle of the day is on Christmas Day.
I think most Americans would understand dinner to be an evening meal, and this seems to be increasingly the case in Britain too. Many will call their evening meal "tea", others "dinner". It seems however, that the word "supper" is in decline.
So I generally agree with Emwue. Duo have probably got it right here.