My Japanese isn't as good as your English, so I'll try to answer in English. I don't know about Raspberry Pi, but on Ubuntu it's a bit limited.
If Mathematica had ??? messages, LANG=ja_JP.UTF-8 export LANG
should turn them on, but that doesn't happen.
On the other hand, it can handle the characters if your keyboard mode is Japanese, regardless of the LANG
setting. I use Anthy with an American keyboard. Input text first appears in the upper left corner of the notebook window, not in the cell I have open. It only appears in the cell when I accept the interpretation with ENTER.
Can you type Japanese into other applications on your Raspberry Pi?