How do I get subtitles on a movie I rented?

iTunes Store Movie Rental FAQ - http://support.apple.com/kb/HT1657 (Now replaced by: About renting movies from the iTunes Store - http://support.apple.com/en-us/HT201611 with much of the same information but not all.)

"Do rented movies have subtitles or closed captioning ?

Some movies do have subtitles and/or closed captioning. Movies containing subtitles will be noted either in the movie title, or within the Plot Summary of the movie. Those that have closed captioning will have the Closed Captioning logo on the movie page: CC"

As with anything you buy these days from anybody, read the "box". If it doesn't say there are subtitles then don't assume there are. There are many of us resident in the USA who speak German, for example. 🙂 There used to be a video rental place near here that had exclusively foreign films and not all of them had subtitles, you had to read each box to be sure if you didn't speak Farsi or Urdu or whatever.