i presume you're using smbx2, which doesn't support non-ascii characters in vanilla textboxes. the best solution i can think of personally is to use a custom textbox library (
littledialogue comes to mind) and create your own font graphic and configuration for japanese characters (though if one already exists someone may be able to point you to it).
however, i wouldn't recommend doing this until you've finalized the entire script for your episode and translated it, to save yourself the headache of drawing and configuring characters which you might not end up using.