We found that making reservations ahead of time outside of Seoul was thankfully not necessary (at least in July when we were there). People at the hotels did not speak English, so reservations would have been very difficult. What we did do was to go to the nearest tourist info centre when we arrived in a city and have them call the places we were considering.
Tourist information offices are clearly signed in English and pretty much anyone who sees you looking lost (and foreign) will direct you to the nearest one. There always seemed to be one very near to the train/bus station and they all have someone who speaks good English. They were very helpful and it was nice to know that there would be a room after we walked over to the hotel!
Seoul is the only place we had trouble finding a room without a reservation – but people who work in hotels there speak English and you can reserve by email at the hostel we stayed at – the Holiday In Korea.