Regarding my portable SNES reader, I added Japanese support using the Genitop GT20L16J1Y FontROM chip. It uses SPI and integrates seamlessly with the Adafruit GFX library.
With the available code/library, Zenkaku works perfectly but Hankaku needs some tweaking. SFC carts only use JIS X 201 Hankaku but the FontROM uses a different ASCII map. I need to figure out how to map the JIS X 201 codes to the FontROM ASCII or JIS X 208 tables. BTW, Zenkaku appears to only be used by some Satellaview ROMs.
I'll need to change my board layout to add the FontROM chip and some additional components as it uses 3.3V.
Take Care!