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i18n

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UpdatedJuly 13, 2026 at 05:31

Internationalization for catalogs, ICU messages, Intl formatting, locale routing, language switching, localized validation, and RTL. 日本語の依頼例:「多言語対応」「翻訳」「ロケール切替」「日付や通貨表示」「RTL対応」。

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Syo-M/codex-frontend-skills

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SKILL.md

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name	i18n
description	Internationalization for catalogs, ICU messages, Intl formatting, locale routing, language switching, localized validation, and RTL. 日本語の依頼例:「多言語対応」「翻訳」「ロケール切替」「日付や通貨表示」「RTL対応」。

Internationalization

No hardcoded user-facing strings

Every user-visible string comes from a message catalog keyed by ID — never a literal in JSX. This is the one rule that, if skipped, makes everything else impossible to retrofit.
Use the framework-idiomatic library: next-intl (Next.js App Router), react-i18next / react-intl (Vite SPA), Astro's i18n routing + a catalog. One per repo (dependency ask-first rule applies).
Keys are semantic and namespaced (checkout.payment.submit), not the English text. Keep catalogs shallow (≤3 levels) and run a missing/orphaned-key check in CI (i18next-parser --fail-on-update, or the library's extractor) — wire it as a gate, don't rely on memory.
Translator-facing context: provide a description for ambiguous keys; never concatenate sentence fragments across keys (word order differs per language).

Plurals, gender, interpolation — ICU MessageFormat

Never build plurals with count === 1 ? 'item' : 'items' — many languages have 3–6 plural categories. Use ICU {count, plural, ...} and let the library + locale data decide.
Interpolate variables through the catalog ({name}), never via template-string concatenation around a translated fragment.
Rich/linked text: use the library's component interpolation (<Trans> / t.rich), not dangerouslySetInnerHTML on a translated string (see frontend-security).

Formatting — always `Intl`, never hand-rolled

Dates/times: Intl.DateTimeFormat with an explicit locale and time zone. Store/transport UTC; render in a deliberately chosen zone; label it when ambiguous (consistent with data-viz).
Numbers/currency/units/lists: Intl.NumberFormat (with style: 'currency' + currency code), Intl.ListFormat, Intl.RelativeTimeFormat. No manual thousands separators, decimal marks, or "3 days ago" strings — these differ per locale.
Currency: format with the data's currency, not the UI locale's; never assume $.

Routing & locale negotiation

Locale in the URL (/ja/... subpath, or domain) so pages are linkable, cacheable, and indexable — not only a cookie/header. Emit hreflang alternates and set <html lang> (and dir) per request.
Negotiate initial locale from the URL → stored preference → Accept-Language, with a sane default; let users switch explicitly and persist it.
SSR/RSC: load only the active locale's catalog on the server and pass it down; don't ship every language's messages to the client (bundle bloat). Code-split catalogs by locale/route.

RTL & layout

Drive direction from data: <html dir="rtl"> for RTL locales; never hardcode LTR assumptions.
Use CSS logical properties (margin-inline, inset-inline-start, text-align: start) through the active styling approach. Physical left/right assumptions break RTL.
Mirror directional icons (arrows, chevrons) in RTL; don't mirror logos or media. Test one RTL locale (e.g. ar) as a first-class case, plus a pseudo-locale for length/encoding.

Locale-aware input & validation

Accept locale-formatted input (decimal comma, native digits, local date order) and parse to a canonical form before validating with zod (see frontend-security) — validate the parsed value, store canonical.
Names/addresses/phone: don't assume Western structure (first/last, state/ZIP). Keep fields permissive; avoid regex that rejects valid non-Latin input.
Sorting/search: locale-aware collation (Intl.Collator), not byte order.

Testing

Pseudo-localization (accented + padded strings) in a story/build catches truncation, clipping, and hardcoded strings early (pairs with visual-regression over both LTR and one RTL locale).
Stories cover at least one non-English and one RTL locale for layout-sensitive components (see storybook).

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Frontend/BFF security for forms and external input, auth, cookies, HTML, URLs, fetch, CORS, webhooks, uploads, secrets, logging, dependencies, and LLM-bound content. 日本語の依頼例:「認証実装」「問い合わせフォーム」「外部API」「webhook」「ログ」「LLMに渡す」「依存追加」。

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Repository policy design or changes: CI, releases, dependency or license policy, secrets scanning, protected-branch/PR rules, and performance budgets. Not for routine metadata edits such as a one-line .gitignore change. 日本語の依頼例:「CI/CD設定」「リリース手順」「依存ライセンス」「ブランチ/PR運用」。

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Next.js-specific App Router work: Server/Client Components, data fetching, caching, Server Actions, route handlers, metadata, and env vars. Not for framework-neutral styling or tests. 日本語の依頼例:「Next.jsで実装」「Server Action」「RSC」「ルートハンドラ」「キャッシュ」。

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Pre-merge verification covering scope, typecheck, lint, affected tests, security, accessibility, and a single pass or fail report. 日本語の依頼例:「出荷前チェック」「マージ前確認」「完成か確認」「PR前チェック」。

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React component, hook, state, effect, and rendering conventions. Use for React implementation or re-render and useEffect debugging. 日本語の依頼例:「Reactコンポーネント」「カスタムフック」「状態管理」「再レンダリング」「useEffect修正」。

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Accessibility implementation and review for semantic HTML, keyboard use, focus, ARIA, contrast, forms, and screen readers. Use for UI components or accessibility fixes. 日本語の依頼例:「アクセシビリティ対応」「キーボード操作」「フォーカス管理」「スクリーンリーダー対応」。

2026-07-130

name	i18n
description	Internationalization for catalogs, ICU messages, Intl formatting, locale routing, language switching, localized validation, and RTL. 日本語の依頼例:「多言語対応」「翻訳」「ロケール切替」「日付や通貨表示」「RTL対応」。

Internationalization

No hardcoded user-facing strings

Every user-visible string comes from a message catalog keyed by ID — never a literal in JSX. This is the one rule that, if skipped, makes everything else impossible to retrofit.
Use the framework-idiomatic library: next-intl (Next.js App Router), react-i18next / react-intl (Vite SPA), Astro's i18n routing + a catalog. One per repo (dependency ask-first rule applies).
Keys are semantic and namespaced (checkout.payment.submit), not the English text. Keep catalogs shallow (≤3 levels) and run a missing/orphaned-key check in CI (i18next-parser --fail-on-update, or the library's extractor) — wire it as a gate, don't rely on memory.
Translator-facing context: provide a description for ambiguous keys; never concatenate sentence fragments across keys (word order differs per language).

Plurals, gender, interpolation — ICU MessageFormat

Never build plurals with count === 1 ? 'item' : 'items' — many languages have 3–6 plural categories. Use ICU {count, plural, ...} and let the library + locale data decide.
Interpolate variables through the catalog ({name}), never via template-string concatenation around a translated fragment.
Rich/linked text: use the library's component interpolation (<Trans> / t.rich), not dangerouslySetInnerHTML on a translated string (see frontend-security).

Formatting — always `Intl`, never hand-rolled

Dates/times: Intl.DateTimeFormat with an explicit locale and time zone. Store/transport UTC; render in a deliberately chosen zone; label it when ambiguous (consistent with data-viz).
Numbers/currency/units/lists: Intl.NumberFormat (with style: 'currency' + currency code), Intl.ListFormat, Intl.RelativeTimeFormat. No manual thousands separators, decimal marks, or "3 days ago" strings — these differ per locale.
Currency: format with the data's currency, not the UI locale's; never assume $.

Routing & locale negotiation

Locale in the URL (/ja/... subpath, or domain) so pages are linkable, cacheable, and indexable — not only a cookie/header. Emit hreflang alternates and set <html lang> (and dir) per request.
Negotiate initial locale from the URL → stored preference → Accept-Language, with a sane default; let users switch explicitly and persist it.
SSR/RSC: load only the active locale's catalog on the server and pass it down; don't ship every language's messages to the client (bundle bloat). Code-split catalogs by locale/route.

RTL & layout

Drive direction from data: <html dir="rtl"> for RTL locales; never hardcode LTR assumptions.
Use CSS logical properties (margin-inline, inset-inline-start, text-align: start) through the active styling approach. Physical left/right assumptions break RTL.
Mirror directional icons (arrows, chevrons) in RTL; don't mirror logos or media. Test one RTL locale (e.g. ar) as a first-class case, plus a pseudo-locale for length/encoding.

Locale-aware input & validation

Accept locale-formatted input (decimal comma, native digits, local date order) and parse to a canonical form before validating with zod (see frontend-security) — validate the parsed value, store canonical.
Names/addresses/phone: don't assume Western structure (first/last, state/ZIP). Keep fields permissive; avoid regex that rejects valid non-Latin input.
Sorting/search: locale-aware collation (Intl.Collator), not byte order.

Testing

Pseudo-localization (accented + padded strings) in a story/build catches truncation, clipping, and hardcoded strings early (pairs with visual-regression over both LTR and one RTL locale).
Stories cover at least one non-English and one RTL locale for layout-sensitive components (see storybook).

i18n

Internationalization

No hardcoded user-facing strings

Plurals, gender, interpolation — ICU MessageFormat

Formatting — always Intl, never hand-rolled

Routing & locale negotiation

RTL & layout

Locale-aware input & validation

Testing

More from this repository

More from this repository

Internationalization

No hardcoded user-facing strings

Plurals, gender, interpolation — ICU MessageFormat

Formatting — always Intl, never hand-rolled

Routing & locale negotiation

RTL & layout

Locale-aware input & validation

Testing

Formatting — always `Intl`, never hand-rolled

Formatting — always `Intl`, never hand-rolled