I plan to travel in South Korea's countryside by car in May and June. I only speak English. Will language be an issue when booking a hotel (I'll probably only book a few days ahead of time), buying food, or driving to the locations I'm aiming for?
It'd be helpful if you indicated which locations you plan to visit as the situation can vary depending on whether it is a popular tourist destination or not. But here's more or less a general answer:
Unless if you plan to drive to a remote location with few population, booking a hotel won't be a problem since majority of hotels have employees that can speak good enough English for you to get by.
Driving won't be too much of an issue, as nearly all road signs include their English names . For other signs that's not written in English, the image is descriptive enough to figure out its meaning but it is probably wise to read up on several road signs you might encounter. If you are renting a car, GPS is definitely recommended. Just be sure to ask if they have an English version (Renting car companies typically do).
Food is a different story unless if you only plan to visit fast food places like Mcdonalds or restaurants that serve Western food. If you wanted to try some authentic local Korean food, it will be very helpful to pick up on at least few common phrases (I.E: 물좀 주세요 (Can I get some water), 반찬 주세요 (Can I get some side dishes [Banchan]), as restaurant owners or employees typically speak little to no English.