fix(theme-translation): complete Japanese theme default translation

[pasora]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

I'd like to complete the Japanese default translations before proceeding to the Crowdin works.

Test Plan

Test links

Deploy preview: https://deploy-preview-8075--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 82	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 79	🟢 100	🟢 100	🟢 100	🟢 90	Report

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[slorber]

LGTM thanks 👍