It seems that those Korean comments are mostly on the English version of the site. (I checked two videos and this was the case with both).
So probably people are just insensitive and triggered that someone is not using English (because it's the English site) or Japanese (people writing in Japanese on the English site is of course okay because weebs).
But I suppose it's understandable. I think in the first place nnd made different languages sites so that comments in different languages don't mix and having a site for multiple languages makes it less cluttered and displays important info (i.e. in the right language) only. After all reading the comments on screen may be a big part of the experience for some, and too many korean comments may ruin it.
Well imo if the portion of Koreans using nnd is growing, they should and maybe will open a Korean site. But as far as I've seen there aren't a lot of Korean comments on screen, and imo people are just being oversensitive about it and paranoid that Koreans will invade their precious English space. ^^'