why is it wrong to say after THE lunch? it does sound kind of strange in English, but it's not necessarily wrong
Report it! "The lunch" is an acceptable phrase in English, since lunch can sometimes be thought of less of an abstract time-of-day but also as a concrete event, similar to a meeting or class.
Do you have to say 'de lunch' in Dutch though? Or can it just be 'na lunch'?
Yes, de is required.
What is it with the word order: place before time?