Translate the README into Japanese (README_JA.md)

We'd love a community-maintained Japanese README! This is a pure documentation PR — nothing to build, no program logic involved.

What to do

1. Create README_JA.md translating the English README.md — the English version is canonical.
2. Open the file with a one-line note in Japanese: this is a community-maintained translation and may lag behind the English README.
3. Keep code blocks, URLs, badges, and images exactly as they are — translate prose only. Documentation links stay as-is (the docs site ships en/zh/fr/ru).
4. Add 日本語 to the language switcher row near the top of the existing README files (README.md, README_ZH.md, README_RU.md, README_FR.md, plus any other community translations present).

Acceptance criteria

• A complete, fluent translation — please write or thoroughly review it yourself rather than submitting raw machine output.
• Switcher row updated in the existing READMEs.

Comment below to claim. PRs target the dev branch (see CONTRIBUTING.md — it has a short section on README translations).