Well, heck, look at how many different languages in which your local DMV offers both the handbook and the written exam. I'm not sure if there's a difference among individual states, but around here, I think there are at least 5 or 6 handbooks. They'll even provide an interpreter if they don't have an exam in your native language. Also, with regards to recognizing road signs, I don't recall having to be able to identify (or follow) street signs and you can easily teach someone the "major" directional signs by looking for shape and or color, so being able to read English isn't all THAT much of a factor. After the test, they can forget about them since hardly anyone else pays attention to STOP or YIELD signs anyhow, so they'll fit right in.

I'm not sure a navigational system would've helped that couple...or are some of the systems available in different languages?