"We sometimes forget about the lunch."
Translation:Czasem zapominamy o obiedzie.
No, you cannot assume a specified lunch when the source is unspecific. Nouns only become soecified when they can be deduced as a property of something else, or if we directly specify it. One of the most difficult things about such a course with no storyline is trying to guess some random decisions by the course creator. Therefore, the use of 'the' here merely further undermines confidence in the course.