We have traveled Chile for a month. If you don't speak Spanish you will at times have difficulties communicating. Not everybody speak English. A lot however do speak English and the are also quite a lot who speak German and French. Even if people don't speak English they often remain quite friendly and you can go a long way with foot and hand language. Often someone is called who does speak English. An English/Spanish dictionary does help but be aware that some distinct differences in similar words exist. Best is to try to pick up local words. You should definitely go for a "completos", which is a hotdog with guacamole sauce. Having said this all, I would not worry much. Chile is a great country to travel through.
I would advise against renting a car only. The buses in Chile are quite comfortable, some companies even offer lie flat seats for not-too-expensive fares. We traveled long distances by bus, but rented cars at location to explore. I did not bring an IDP; my EU license was widely accepted.
When you do rent a car to explore, make sure your contract allows getting on unpaved roads. In some areas more the half of the roads are unpaved, limiting your options drastically. It also helps to bring a GPS. We had a Garmin etrex vista with maps downloaded from open street view.