The tours are in German. But a lot of it is statistical, so you won't (at least I wasn't) left in the dark. You get to SEE interesting sections of the airport. The best part IMHO, is the stop on the tarmac where two runways cross each other and you see planes up close.
I went on a Sunday, lots of people--if you can do it on Saturday, it may be more relaxed and you many not have to wait too long after you purchase your ticket.
PS: To get to buy the tour ticket you must first buy a flight deck entrance (free if you have a Zurich card--covers museums, mass transit also). I'm not sure if you get credit toward the tour (if you don't have a ZC).