As a non-native English speaker, English is always big burden at work place regardless working at foreign companies or Korean companies. I strongly recommend Don Lee's English coaching program.
The biggest reason for my recommendation is Don's team is very well organized and customized for all clients. However, I don't think Don Lee's program is not only for improving English, but it is more for English coaching. Again, it is coaching.
For me, I learned how to develop business strategies, communication skills and detail action plans from Don's team. Those who want to learn not only English but also business capabilities, I urge you should join Don's English coaching programs immediately.