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

---

💡 Configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[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