I suggest you go to the higher level (ie South Korea) and do a search on Yeosu.
I think hotels will have SOME English but don't expect much. You nay be better to stay in or around Gwangju and travel.
I also will be attending expo at Yeosu.
>Any idea how many days I should plan to be there to see everything?
My intention is 6 days, based on spending 6 days in the similarly sized expo in Zaragoza in 2008. I didn't see every pavilion, but felt content with what I did see.
>Do they speak English?
Usually English is a very useful language at expos, as it's a common second language and used for chat between different country staff. The theme pavilions are host country supplied, so in host language, but often designed with the hope of being understood by most people.
I've always found it useful to learn or brush up on the host country language, and may come to regret not having done so this time!