Hello,
This is a problem. Skin developers choose whatever fonts (.ttf, .otf, ...) they like best. But, those files don't necessarily have entries for all languages.
The default skin uses the default font for the main window. The file is FreeSans.ttf. It works for Japanese but not for Korean.
An easy way to solve the problem here is to download a font that supports Korean. A quick search for free fonts on the Internet gives for instance the site
https://fr.fonts2u.com/ and the first font found for Korean is CODE2000.TTF.
You can download the file, put it in the same directory as FreeSans.ttf, edit the skin config file (theme.xml), add "<Font id="defaultfont" file="CODE2000.TTF"/>", rezip the vlt file and it works.
You could also just rename CODE2000.ttf as FreeSans.ttf. and it can also work, but that is really ugly.