"He is eating lunch."
Translation:On je obiad.
Many Poles just use 'lunch" for the modern concept of "lunch" and Duo accepts that here as well. "On je lunch" is accepted. Obiad is somewhat different from an American or British lunch as it is the heavy main meal of the day and equates more to an early dinner. From Wiki "In Poland the main meal of the day (called "obiad") is traditionally eaten between 12:00 and 16:00, and consists of a soup and a main dish. Most Poles equate the English word "lunch" with "obiad" because it is the second of the three main meals of the day; śniadanie (breakfast),obiad (lunch/dinner) and kolacja (dinner/supper). There is another meal eaten by some called "drugie śniadanie", which means "second breakfast". "Drugie śniadanie" is eaten around 10.00 o'clock and is a light snack, usually consisting of sandwiches and / or salad or a thin soup." From my family experience 'obiad' it is generally eaten around 14:00-16:00 particularly on weekends with the whole family.
As a Pole and Polish language native speaker I have never met anyone who uses "lunch" as a dinner (obiad). Never. Lunch is always "drugie śniadanie", although "lunch" is very rarely in use.