Keep in mind that this USB loader was originally designed to work exclusively with USB drives, which is why it's called USB Loader GX. However, people kept asking for the ability to play Wii games from an SD card, so when I implemented "SD card mode" I deliberately made it skip all USB mounting logic because it was easier to implement and it also speeds up the boot process due to the lack of checks and countdowns.
Large SD cards can often be bought cheaply during sales, and the next update will also add support for large (3TB+) USB drives, so I personally wouldn't want to change the way SD card mode works.
I normally recommend permanently assigning modded games a custom ID.
https://gbatemp.net/posts/10610263
What Japanese games are you trying to play?
The loader is supposed to attempt to get the title from the banner file in your systems language, but since the text is blank for you, it seems like it's using Japanese, which a non-Japanese Wii can't display.