Japanese Yokai Watch Busters Not Working

Assuming you have Luma3DS CFW, you may have to use LumaLocaleSwitcher or LumaLocaleRegionAutoSetter in order to change the region and language of the game to Japanese and allow your system to run it properly. You can do it to cartridge games too.

 

You probably just need to force the correct region/language in Luma. Sometimes out of region (especially Japanese) games won't run because they are looking for corresponding files to the console's region/language and aren't made to fall back to Japanese when those files are not found.