Implement & Test: Repack files. Test every screen — text overflow (Japanese wider), font fallback (need JP font: Noto Sans JP, M+, Rounded M+, Shippori Mincho), line breaking (Kinsoku Shori), ruby/furigana.
QA: Native speaker playthrough.
Resources
Fonts (Free Commercial): Google Fonts (Noto Sans JP, Noto Serif JP, M PLUS, Zen Maru Gothic), fonts.google.com.