1. $VTLANG_XXXXX must be pre-defined in grub menu.txt https://github.com/ventoy/Ventoy/tree/ma.../grub/menu
So if you set current Language to Korean, then you must firstly define VTLANG_Ubuntu_BIOS/VTLANG_Ubuntu_UEFI ... in ko_KR.json
2. For grub variable expansion, you should use "$VTLANG_XXX" not '$VTLANG_XXX', that is to use double quotes instead of single quotes.
So if you set current Language to Korean, then you must firstly define VTLANG_Ubuntu_BIOS/VTLANG_Ubuntu_UEFI ... in ko_KR.json
2. For grub variable expansion, you should use "$VTLANG_XXX" not '$VTLANG_XXX', that is to use double quotes instead of single quotes.