Improve Japanese and Simplified Chinese documentation quality #279

Background

Improve the Japanese and Simplified Chinese documentation so it reads naturally and consistently, while preserving the meaning of the English canonical documents. The recently improved Korean documentation may serve as a style reference, but the English source remains authoritative for meaning.

Scope

Only these eight files in this repository:

• Japanese: README.ja.md, RELEASING.ja.md, SECURITY.ja.md, plugins/session-peer/README.ja.md
• Simplified Chinese: README.zh-CN.md, RELEASING.zh-CN.md, SECURITY.zh-CN.md, plugins/session-peer/README.zh-CN.md

For both root localized README files, center the title, badges, language selector, and short introduction inside <div align="center">, closing the wrapper before the body.

Acceptance criteria

• Improve natural phrasing and consistency of terminology and style across each language.
• Verify against the English canonical documents that feature descriptions, installation scope, versions and numbers, commands and flags, identifiers and placeholders, code examples, security limitations, release evidence, and operational boundaries retain their meaning.
• Preserve Markdown structure, code blocks, links, and anchors; pass the local documentation checks and CI.
• Do not change the Korean documentation, English canonical documents, runtime code, or operational settings.

This issue covers Japanese and Simplified Chinese documentation quality only. It does not change the scope of the existing Korean translation issue or PR.