Add Japanese translation

[nunawa]

Japanese translation seemed to be missing, so I added it!
I referred to #437 and #846.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.46%. Comparing base (6444260) to head (1fbe57d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #909   +/-   ##
=======================================
  Coverage   82.46%   82.46%           
=======================================
  Files          85       85           
  Lines        4682     4682           
=======================================
  Hits         3861     3861           
  Misses        821      821           

Flag	Coverage Δ
Linux	82.16% <ø> (+0.04%)	⬆️
macOS	82.33% <ø> (ø)

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