Indeed, lots of humor from movies is untranslatable, but here is one example of a dumb joke that is:

In the movie Beavis and Butthead do america there is a scene in which the main characters are trapped in the trunk of a car. Butthead finds a jack and starts 'jacking' away.
Hey Beavis, I'm jacking !

This translates quitte nicely to Dutch:
Hey Beavis, ik ben aan het krikken !!

Which means about the same thing, except where jacking is performed solo and 'krikken' involves another person (preferably of the opposite sex). And a 'krik' is the dutch word for a car's jack.

Offcourse in most languages a lot of words can be used to carry a second, naughty meaning.