Do Korean Speak English Well?
It varies from individual to individual. If your mother tongue is English, generally speaking, it would be not easy for you to communicate in English when traveling and staying in S.Korea.
Generally, Korean are not good at communicating in English. Even though most of Korean started studying English from their primary schools, English classes in schools are not enough for them to have good command of English. Main reason for this is that English education in schools still revolves around grammar knowledge and reading skills.
Even though many Korean including young students are actively taking private lessons to improve their speaking and listening skills, it is not easy for many Korean English learners to improve their practical English abilities mainly due to the fact that Korea is EFL(English as a Foreign Language) environment, which makes it difficult to practice what they learned.
Another thing I want to mention is that many Korean tend to be shy when foreigners approach to ask directions or some other questions.
Therefore, I personally recommend that you prepare your travel to S.Korea in many ways meticulously, such as accommodation, transportation, foods, places to visit, how to get there etc.
Even though I say that Korean are not good at English in general, there are still lots of Korean who can communicate well in English, and Korean are kind to foreigners. While traveling to S.Korea, if you need some help, just approach Korean with smile, then the Korean will try to help you with kind attitude.