I don't know of any shops offhand that cater specifically to the foreign market or offer English service. However, many shops do have staff who can speak English - varying degrees of fluency. It's mostly luck of the draw.
Hopefully someone who has married recently will have some specific suggestions for you. However, you might also want to check out some shops yourself - in Sinchon, near Ewha Womans University is a street full of wedding gown shops.
If you do find one with a dress you like but there is no English speaker, you can contact BBB and one of their volunteers will interpret for you while you are in the shop. Call 1588-5655 - they offer translation in 17 languages. See
http://www.korea4expats.com/service-BBB---Free-Interpretation-Help-in-17-Languages-884.html