Language / i18n Configuration

Closes: Issue #173 — English version / how to set the global output language.

User-facing responses (skill outputs, slash commands, hook messages) follow the explicit session or project language. When no language is configured, the default is English. Use Japanese only when explicitly requested via one of the three mechanisms below.

This file is the SSOT. CLAUDE.md Language section is a short pointer to here. The README links here under "Language".

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How to switch language

There are three ways to set the language. Resolution precedence, highest first:

1. A per-message session instruction (you explicitly ask the agent to switch).
2. The project config .claude-code-harness.config.yaml (i18n.language).
3. The CLAUDE_CODE_HARNESS_LANG env var.

If none are set, the default is en. Note that the project config takes priority over the env var: when i18n.language is present it wins, and CLAUDE_CODE_HARNESS_LANG only applies as a fallback for projects that do not set it.

1. Project-level config — .claude-code-harness.config.yaml

For a per-project setting that persists across sessions, set the language in the harness config file the runtime reads, .claude-code-harness.config.yaml:

i18n:
  language: en   # en | ja (default: en)

This applies to every Claude Code session run inside the project, and the language directive is re-injected on every turn so responses stay in the configured language. Commit this file to make the choice repository-wide.

Note: harness.toml is the v4/v5 build source consumed by harness sync; it does not currently carry the runtime language setting. Set i18n.language in .claude-code-harness.config.yaml.

2. Per-session env var — CLAUDE_CODE_HARNESS_LANG

For a project that does not pin i18n.language, export the env var before launching Claude Code to choose the language for that session:

export CLAUDE_CODE_HARNESS_LANG=ja
claude            # this session uses Japanese

Or inline:

CLAUDE_CODE_HARNESS_LANG=ja claude

If the project config sets i18n.language, it wins over this env var. To override a config-pinned language for a single session, use a per-message session instruction (below).

3. Per-message session instruction

For ad-hoc switching mid-session, tell the agent in your own words:

> 以後は日本語で返答してください

> Please respond in English from now on.

This wins over both .claude-code-harness.config.yaml and CLAUDE_CODE_HARNESS_LANG for the rest of the session.

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What does NOT change

• Machine-readable values (JSON schema fields, contract verdicts, log keys) stay English regardless of the language setting.
• Source code identifiers, file names, commit hashes, command names stay as-is.
• Conventional Commits prefixes (feat: / fix: / docs: ...) stay English.

If you see Japanese in a place that should be machine-readable (e.g. a JSON contract field), please open an Issue — that's a bug, not an i18n setting question.

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Supported languages

Code	Status
en	Supported (default)
ja	Supported
other (zh, ko, de, etc.)	Not officially supported — session instruction can request best-effort, but skill outputs may include English fallbacks

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Related

• CLAUDE.md § Language (short pointer to this file)
• README.md § Language (link to this file)
• .claude-code-harness.config.yaml (i18n.language, read by the runtime and hooks)