Garbled texts on WSL GUI application

That's not really garbled – that's just how Linux programs display characters that they don't have any fonts for. The hexadecimal digits inside each box correspond to Unicode codepoints for the character. For example:

• 한 is U+D55C
• 국 is U+AD6D

You should install more fonts in your Linux distribution. If you're using Ubuntu, sudo apt install fonts-noto-cjk. The Noto font set supports many languages.

Or if you know that a specific font is needed, you can copy .ttf or .otf files to ~/.local/share/fonts/ in Linux. You could even copy all the Windows fonts from C:\Windows\Fonts if you want.