I'm no programmer, but Mala is pulling the info from mame.xml so you'd probably start there.
The names in mame.xml would have to be in Japanese, but the tags that Mala uses to locate the names would still have to be in English.
You could try to modify one by hand and see if it works.
mlgconverter.exe from this thread will allow you to convert a Mala gamelist to a .txt file and back.
http://forum.arcadecontrols.com/index.php/topic,101104.0.htmlSo you might try modifying the name there.
I have no idea if either of those will work or if it is possible.
You might be better off finding a front-end written natively in Japanese or a version of MAME that already includes a Japanese user interface.