I challenged to modify the firmware too.
From some of the mod firmware, I have determined that the language setting is done with address (301AE).
I confirmed Italian (0A) and Thai (0C).
From the order of the languages (in firmware file), it was predicted that the value of Japanese is 0B.
After installing mod fw, the day of the weeks display on the clock screen became Japanese.
It seemed that Japaneseization was successful.
土 (This is a sign like ±) means Saturday in Japanese.
However, all characters disappeared except for the clock screen.
There seems to be some missing elements to display Japanese in items such as weather, heart rate and steps.
In my ability this seems to be a limit.
By the way, Japanese was displayed only when returning to English firmware.