I used to work with a couple Czech dudes. One day my coworkers and I were badgering the one dude to tell us a Czech joke. He was pretty reluctant because he said he could only really think of one joke but wasn't sure it would translate well. When he finally told us the joke he got us with this masterpiece.
Two balloons are floating along, one says to the other and hey look a cactus.
All of us were confused by this, he told us it was much funnier in Czech because balloon and cactus sound similar so it's a pun. So we had him tell us the joke untranslated in Czech and balloon and cactus sound nothing alike.
I'm still not sure if this dude was fucking with us.