Hi, Edith.
Usually, as teamwork goes, you would have several Korean-to-English subtitlers, which don’t necessarily have to be fluent in English, do their job first. Then one or more Editors would come in to fix the very things you mention. Sometimes the teams are small and all the responsibility of perfecting the language lies with just one person, one Editor. If the team is bigger (usually when the project is more popular), it will have several Editors who specialise in particular corrections and edits.
Editors are usually set from the start of the project. You can however, ask the Manager of that particular channel to do post-editing, if something is really bugging you.
From what I understand (having never been an Editor myself, mind you), is that Editors usually don’t like to change the subtitles to make them just tiny bit better (ie. less formal). Reason being they would like the original subtitler to be given credit for what is mostly their own work. Only in cases where the translation is very unnatural or the meaning is ambivalent will the Editor intervene. However, there are no firmly set rules here and each Editor works a bit differently.
With your expertise, though, and particularly if you have time, please consider becoming an English editor for some of Viki’s on-going projects. Help is always needed! 