Update Japanese translation

[gemmaro]

Hello RubyGems,

This updates Japanese translation (continuation of #4779).
Each commit adds changes corresponding to those made to the respective English/Japanese file (en.yml/ja.yml), as specified in the commit subject.

Thank you,
gemmaro.

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TODOs to be ready for review.

• Self-check the translation by looking at the way the messages are called.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.23%. Comparing base (e5b0e6e) to head (e2e931d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5272      +/-   ##
==========================================
- Coverage   96.88%   94.23%   -2.65%     
==========================================
  Files         457      457              
  Lines        9576     9635      +59     
==========================================
- Hits         9278     9080     -198     
- Misses        298      555     +257     

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