docs: add Japanese (日本語) README

[RKasai127]

Description

Adds a Japanese translation of the project README as README.ja.md.

Spec Kit already has a Simplified Chinese translation (#3740) but no Japanese one, so this follows the same pattern.

New file

• README.ja.md — a full Japanese translation.
• README.md — added 日本語(japanese) to the language switcher under the badges.
• README.zh-CN.md — added 日本語(japanese) to its language switcher as well.

All code blocks, CLI commands, slash-command names, badges, image paths, and link URLs are unchanged — only prose and inline shell comments were translated.

Testing

This is a documentation-only change with no impact on CLI behavior or slash commands, so the checkboxes below don't apply.

• Tested locally with uv run specify --help — N/A (docs-only change)
• Ran existing tests with uv sync && uv run pytest — N/A (docs-only change)
• Tested with a sample project (if applicable) — N/A

Manual verification performed instead:

• Confirmed every Table of Contents anchor resolves to the correct (translated) heading.
• Confirmed section count, order, and links match the English README 1:1.

AI Disclosure

• I did use AI assistance (describe below)

I used Claude Code (model: Claude Sonnet 5, interactive mode) to produce an initial draft translation, then reviewed and corrected it myself as a native Japanese speaker — rewriting phrasing to read naturally rather than as a literal machine translation, and checking terminology and links against the English source section by section.

Note

If documentation/translation contributions like this aren't something the project wants to take on right now, please feel free to close this — no hard feelings, and thanks for all the work maintaining Spec Kit!

[Copilot]

🟡 Changes recommended

The translation contains inconsistent SDD terminology and several formatting errors.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Adds a Japanese README and connects it to the existing language switchers.

Changes:

• Adds the Japanese translation.
• Adds Japanese navigation links to existing READMEs.

File summaries

File	Description
README.ja.md	Adds Japanese documentation.
README.md	Links to the Japanese README.
README.zh-CN.md	Links to the Japanese README.

Review details

Suppressed comments (3)

README.ja.md:78

• The sentence is ended before the command reference and then punctuated again, unlike steps 1–4. Move the command parenthetical before the sentence-ending mark.

5. **収束させる(Converge)**: 実装が仕様・計画・タスクと一致しているかを検証し、ズレを収束させます。(`/speckit-converge`)。

README.ja.md:441

• The linked methodology is called 仕様駆動開発 throughout this translation (for example, lines 57–59 and 380), so 規範駆動開発 is an inconsistent and meaning-changing translation here.

- **[規範駆動開発の完全な方法論](./spec-driven.md)** - プロセス全体を深く掘り下げる

README.ja.md:448

• The support text again switches from the established 仕様駆動開発 terminology to 規範駆動開発, which refers to norms rather than specifications.

サポートが必要な場合は、[GitHub issue](https://github.com/github/spec-kit/issues/new) を開いてください。バグ報告、機能リクエスト、規範駆動開発の使い方に関する質問を歓迎します。

• Files reviewed: 3/3 changed files
• Comments generated: 8
• Review effort level: Balanced

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[mnriem]

Thanks for contributing a Japanese translation and reviewing the initial draft yourself. This follows the existing Chinese README approach.

We substantially reorganized the README in #4591 after you opened this PR. Please refresh the Japanese version against current main, preserving the new shared setup, three independent processes, and distinction between terminal commands and skills invoked in agent chat.

Please also add the Claude model(s) and mode/settings used to the disclosure. The extent of assistance and your subsequent review are already clear.

Drafted for @mnriem with assistance from GitHub Copilot (model: GPT-6 Astra; interactive comment drafting).

[RKasai127]

@mnriem

Thanks for the review!

I've updated the AI Disclosure section to include the specific model and mode used.

I've also refreshed the Japanese translation to match the restructured README.

Please take a look.

[Copilot]

🟢 Approval recommended

The documentation is consistent with the English source, with only a non-blocking punctuation cleanup noted.

Review details

Suppressed comments (1)

README.zh-CN.md:17

• This leaves a visible · after the final language option, unlike the English and Japanese switchers. Remove the trailing separator so all three switchers render consistently.

    <a href="./README.ja.md">日本語</a> ·

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

🟢 Approval recommended

The documentation is consistent and complete, with only a minor punctuation nit.

Review details

Suppressed comments (1)

README.ja.md:50

• The sentence uses an ASCII question mark in otherwise Japanese prose. Use the full-width Japanese question mark for consistent punctuation.

すでにコードがありますか?[既存プロジェクト向けガイド](https://github.github.io/spec-kit/guides/existing-projects.html)を参照してください。バージョン固定でのインストール、その他のインストーラー、CI、トラブルシューティングについては[インストールガイド](https://github.github.io/spec-kit/installation.html)を、既存のインストールを更新する場合は[アップグレードガイド](https://github.github.io/spec-kit/upgrade.html)を参照してください。

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation is consistent, reciprocal links work, and technical content remains unchanged.

Review effort: Balanced
Findings: None

[mnriem]

Thanks for refreshing this and for reviewing the translation as a native Japanese speaker. I asked GitHub Copilot to perform a complete side-by-side comparison of README.ja.md against the current English README, and I am relying on that review for the source-fidelity observations below. The commands, links, inline literals, and overall technical instructions were otherwise found to be consistent.

Since your latest update, main gained a Star history section. Could you refresh the Japanese README once more and add the corresponding translated section before Support and contributing, preserving the existing link, image sources, and markup?

The side-by-side review also identified three phrases worth checking:

• README.ja.md:26: “A specification carried through planning, implementation, and convergence” is translated as 計画・実装・収束を経て完成した仕様, which may suggest that the specification itself is completed by those stages. Would 計画・実装・収束まで一貫して活用される仕様 better preserve the intended meaning?

• README.ja.md:48: Please consider translating the repository term integration key as インテグレーションキー or 統合キー, rather than エージェント識別子, because copilot specifically identifies the CLI integration.

• README.ja.md:111: アイデアにコミットする前に may be ambiguous in a software-development document because “commit” can suggest Git. Something like アイデアへの投資を決める前に may more directly convey the English meaning.

Please use your native judgment for the final Japanese phrasing. The missing Star history section is the only blocking issue identified by the comparison.

Drafted for @mnriem by GitHub Copilot (model: GPT-5.6 Sol; interactive mode); comment fully AI-drafted following a complete side-by-side review of the English and Japanese READMEs.

[RKasai127]

@mnriem

Thank you for checking. I've added the corresponding "Star History" section to the Japanese README.
I've also addressed the phrasing points you raised, as shown below. Please take a look.

README.ja.md:26: ...

"活用" (used/leveraged) reflected the actual behavior more accurately than "完成" (completed), so I changed it to 計画・実装・収束まで一貫して活用される仕様.

README.ja.md:48: ...

"統合キー" is technically accurate, but it's not a term commonly used in Japan, so I changed it to 連携キー instead, striking a balance between idiomatic phrasing and technical accuracy.

README.ja.md:111: ...

"投資する価値があるか判断する" in the table (line 28) reads naturally in Japanese, but in the context of the body text (line 111), "投資" felt slightly off, so I judged that 進める reads more naturally there and changed it to アイデアを進めると決める前に.

[Copilot]

Copilot review overview

🟢 Approval recommended

The translated README preserves the source structure, commands, and links without introducing functional issues.

Review effort: Balanced
Findings: None

[mnriem]

Thank you!