the most practical way is to use a free subtitle translation site, such as: https://translate-subtitles.com/ . or search on google with keywords: "translate subtitles online" . just upload the .srt file, then select the desired language .. and automatically download according to the language we choose. it works!!

do we have to extract the existing chinese/korean subs using OCR first ?

if the format is still in the form of .sub and .idx files, of course it must be OCR first.. but if it is in the form of .srt, it can be done immediately.