Picture of the Apple II j-plus screen showing Japanese (KATAKANA) activation.

POKE -16292,0 : activates the KATAKANA font set.

CTRL T : toggles between english and KATAKANA.

POKE -16291,0 : activates inverse english text and turns KATAKANA off.

CTRL T : toggles between regular and inverse text.

The ROMs have been archived and can be found here.

Thanks to Sckop for the information