Proofread README_ja.md: no Japanese speaker has ever reviewed it

Read README_ja.md alongside README.md, focusing on the pitch, engine list, install table, and quick start. Preserve the deliberate abridgement, engine names, and install commands; run bun run readme:check before pushing, then run bun run test as required. Done means the Japanese prose reads naturally and both checks remain green.

README_ja.md has never been read by a Japanese speaker.

It arrived in #317 (commit 8f97127), written by a contributor who does not speak Japanese, and the only reviewers on that PR were the maintainer and Copilot. Every other translation since then got native-speaker review on the PR: Urdu had three readers besides the author (#783), Hindi had two (#845). Japanese had none. The file has been live since 2026-08-08.

So this issue is not a translation task. It is a proofreading pass by someone who reads Japanese.

What to do.

Read README_ja.md (303 lines) against README.md and fix what reads wrong: unnatural phrasing, wrong register, terms that a Japanese developer would keep in English but that were translated anyway, sentences that are grammatical but say something the English does not.

Two ways to contribute, both welcome:

1. Open a PR with the fixes. This is the more useful one and it is what counts for Hacktoberfest.
2. Comment on this issue with a list of what is wrong, if you would rather not touch git. Someone else can then apply it.

Scope, so the diff stays reviewable.

README_ja.md is a deliberate abridgement, not a full translation. README.md is 1053 lines; the five localized files run between 240 and 315. They carry the pitch, the engine list, the install table and the quick start, and they drop the deep sections on purpose. Do not add the missing sections here. If you think a section should be added, say so in a comment and we will open a separate issue for it.

Two things the CI guard will not let you change, and they are not bugs.

scripts/readme-check.mjs runs inside the required Lint, Typecheck and Build check and asserts:

• The engine name set is identical in every localized file and README.md. Do not translate engine names. PostgreSQL stays PostgreSQL.
• Every command in the install table appears verbatim in README.md's. A rewritten command fails the check, including a dropped sudo.

Run bun run readme:check before you push.

Done when the Japanese prose reads as though a Japanese developer wrote it, bun run readme:check passes, and the engine names and install commands are untouched.

A PR must reference this issue; see CONTRIBUTING.md. Repo rules that apply: run bun run test (never bare bun test), and the 100% line-coverage gate must stay green. Local bun run test also needs the helm binary on PATH for unrelated chart tests, see #570 if it bites.