The subway has better English signage, than the metro in Osaka, but English ability/signage on the street is comparable. Of course, at the tourist attractions/tourist areas there will be more of an international presence, therefore more English, but if you are staying in Yeongdeungpo, expect there to be much less English than say Kangnam, or Myungdong.
I recommend looking over
http://wikitravel.org/en/Seoul
Despite what people will tell you, avoid Itaewon, its a foreigner ghetto. If you want to see Seoul, Itaewon isn't it.
If you have any other questions, just ask, I live in Seoul, so I can answer alot of questions.