Basically, Lunch is lunch, obiad is dinner, and kolacja is supper. At least this is what I usually hear from Polish native speaking people. Thinking of it, in Norwegian we also use the word lunch, and not some other fancy native word for the meal around 12 o clock.

Jesteśmy śczęśliwi że przygotowałeś obiad.

many english words have been incorporated into the polish language--for example, there is no direct word for the weekend in polish except for the word itself. same goes for words like lunch. because we do not have a real word for lunch or dinner, just a generalized concept of it as obiad, many polish speakers choose to say lunch for lunch. this of course is also dialectical and regional--it depends on where the polish speaker comes from

See a previous discussion on this. Someone gave a great explanation.