After fiddling around I can see this extension works for different and same language shows, e.g. jpns subs for jpns shows, krn subs for engl shows, but no krn subs for krn shows? Why…?
The problem is likely that Netflix do not allow Korean subs for the movie/series in your country. :-/ Usually the native language is always available, appart from with older Japanese material, I didn’t know it happens for Korean.