Hi Michael,
I thought the movie was so funny! I don't know what your native language is... I will try to make a suggestion. "Fopje" doesn't really exist as a word (at least not in Dutch), I think it is an invention that has to do with "foppen" which means "to fool". "Flauw" means stupid, cheesy. And a "mopje" is a little joke. I just googled Fopje and I see it's kind of an expression in Flemish (the whole expression Fopje flauw mopje), I didn't know (because I'm Dutch), so maybe you can translate it like "Stupid little joke". Or were you looking for something more creative?
Good luck and thanks for the laugh :-)