Okay, quick question. Does the game use a standard NFTR for its font? If so, it might have an English font already built in. If you've confirmed that it doesn't, you might be able to copy the NFTR from another game that does have an English font.
At any rate, if your English font is the same size as the Japanese font, just move it down a couple of pixels so it shares the same baseline as the Japanese font.