Before someone more expert than I comes along I believe that the Japanese-fitted head units will only have Japanese, there is no switching to English.
You can use Google Translate (or Lens come to that) to translate and, for the buttons, you could put fresh labels over the buttons.
However the only permanent solution is to change the head unit, though people have lots of fun with these as they never seem to be a straight swap and the various functions may need particular adaptors to work:
reversing camera,
front side-ways cameras (if fitted)
steering wheel controls,
audio / video out (some Alphards have a separate amplifier, many speakers, middle screens etc.)
hand-free phone use
other stuff ?
i would start with sticker / labels, even if they are just bits of post-it notes, and see how you get on.