added Japanese translations

[donuzium]

Description of what you did:

Updated Japanese translation file in strapi-admin.

My test result

$ yarn test:front
...
Test Suites: 92 passed, 92 total
Tests:       470 passed, 470 total
Snapshots:   11 passed, 11 total
Time:        25.875s
Ran all test suites.
✨  Done in 29.32s.

Thank you for your great work!!

[codecov]

Codecov Report

Merging #6536 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6536   +/-   ##
=======================================
  Coverage   19.85%   19.85%           
=======================================
  Files         857      857           
  Lines       12076    12076           
  Branches     1963     1963           
=======================================
  Hits         2398     2398           
  Misses       8093     8093           
  Partials     1585     1585           

Flag	Coverage Δ
#front	14.66% <ø> (ø)
#unit	41.26% <100.00%> (ø)

Impacted Files	Coverage Δ
...ntent-type-builder/controllers/validation/types.js	52.27% <ø> (ø)
...ages/strapi-plugin-content-manager/services/uid.js	100.00% <100.00%> (ø)

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[soupette]

LGTM!

Thanks for your contribution