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Localization & Internationalization (i18n)

Internationalization (i18n) is the process of designing software so it can be adapted to different languages and regions without engineering changes. Localization (l10n) is the actual adaptation - translating strings, formatting dates and currencies, supporting right-to-left scripts, and handling pluralization rules that vary wildly across languages. This skill gives an agent the knowledge to set up i18n infrastructure, write correct ICU MessageFormat patterns, handle RTL layouts, manage translation workflows, and avoid the common traps that cause garbled UIs in non-English locales.

When to use this skill

Trigger this skill when the user:

• Wants to add i18n/l10n support to a web or mobile application
• Needs to write or debug ICU MessageFormat strings (plural, select, selectordinal)
• Asks about handling right-to-left (RTL) languages like Arabic or Hebrew
• Wants to set up a translation workflow or integrate a TMS (translation management system)
• Needs to format dates, numbers, or currencies for specific locales
• Asks about pluralization rules for different languages
• Wants to configure i18n libraries like react-intl, i18next, FormatJS, or vue-i18n
• Needs to extract translatable strings from source code

Do NOT trigger this skill for:

• General string manipulation unrelated to translations
• Timezone handling without a localization context (use a datetime skill instead)

Key principles

1. Never concatenate translated strings - String concatenation breaks in languages with different word order. Use ICU MessageFormat placeholders instead: "Hello, {name}" not "Hello, " + name. This is the single most common i18n bug.

2. Externalize all user-facing strings from day one - Retrofitting i18n is 10x harder than building it in. Every user-visible string belongs in a message catalog, never hardcoded in source. Even if you only ship English today.

3. Design for text expansion - German text is 30-35% longer than English. Japanese can be shorter. UI layouts must accommodate expansion without clipping or overflow. Use flexible containers, never fixed widths on text elements.

4. Locale is not language - en-US and en-GB are the same language but format dates, currencies, and numbers differently. Always use full BCP 47 locale tags (language-region), not just language codes.

5. Pluralization is not just singular/plural - English has 2 plural forms. Arabic has 6. Polish has 4. Russian has 3. Always use CLDR plural rules through ICU MessageFormat rather than count === 1 ? "item" : "items" conditionals.

Core concepts

ICU MessageFormat is the industry standard for parameterized, translatable strings. It handles interpolation, pluralization, gender selection, and number/date formatting in a single syntax. The key constructs are {variable} for simple interpolation, {count, plural, ...} for plurals, {gender, select, ...} for gender/category selection, and {count, selectordinal, ...} for ordinal numbers ("1st", "2nd", "3rd"). See references/icu-message-format.md for the full syntax guide.

CLDR Plural Rules define how languages categorize numbers into plural forms. The Unicode CLDR defines six categories: zero, one, two, few, many, other. English uses only one and other. Arabic uses all six. Every plural ICU message must include the other category as a fallback. See references/pluralization.md.

RTL (Right-to-Left) layout affects Arabic, Hebrew, Persian, and Urdu scripts. RTL is not just mirroring text - it requires flipping the entire layout direction, swapping padding/margins, mirroring icons with directional meaning, and using CSS logical properties (inline-start/inline-end instead of left/right). See references/rtl-layout.md.

Translation workflows connect developers to translators. The typical pipeline is: extract strings from source code into message catalogs (JSON/XLIFF/PO files), send catalogs to translators (via TMS or manual handoff), receive translations, compile them into the app's locale bundles, and validate completeness. Missing translations should fall back to the default locale, never show raw message keys.

Common tasks

Set up react-intl (FormatJS) in a React app

Install the library and wrap the app with IntlProvider.

npm install react-intl

import { IntlProvider, FormattedMessage } from 'react-intl';

const messages = {
  en: { greeting: 'Hello, {name}!' },
  fr: { greeting: 'Bonjour, {name} !' },
};

function App({ locale }) {
  return (
    <IntlProvider locale={locale} messages={messages[locale]}>
      <FormattedMessage id="greeting" values={{ name: 'World' }} />
    </IntlProvider>
  );
}

Always load only the messages for the active locale to minimize bundle size.

Set up i18next in a Node.js or React app

npm install i18next react-i18next i18next-browser-languagedetector

import i18n from 'i18next';
import { initReactI18next } from 'react-i18next';
import LanguageDetector from 'i18next-browser-languagedetector';

i18n
  .use(LanguageDetector)
  .use(initReactI18next)
  .init({
    resources: {
      en: { translation: { welcome: 'Welcome, {{name}}!' } },
      ja: { translation: { welcome: 'ようこそ、{{name}}さん！' } },
    },
    fallbackLng: 'en',
    interpolation: { escapeValue: false },
  });

i18next uses {{double braces}} for interpolation by default, not ICU {single braces}. Enable ICU MessageFormat with the i18next-icu plugin if you want standard ICU syntax.

Write ICU plural messages

You have {count, plural,
  =0 {no messages}
  one {# message}
  other {# messages}
}.

The # symbol is replaced with the formatted number. Always include other as the fallback category. For languages with more plural forms (Arabic, Polish, Russian), translators add the additional categories (zero, two, few, many).

See references/icu-message-format.md for select, selectordinal, and nested patterns.

Write ICU select messages (gender/category)

{gender, select,
  male {He liked your post}
  female {She liked your post}
  other {They liked your post}
}

The other branch is required and acts as the default. Select works for any categorical variable, not just gender.

Format dates, numbers, and currencies per locale

// Numbers
new Intl.NumberFormat('de-DE').format(1234567.89);
// -> "1.234.567,89"

// Currency
new Intl.NumberFormat('ja-JP', {
  style: 'currency',
  currency: 'JPY',
}).format(5000);
// -> "￥5,000"

// Dates
new Intl.DateTimeFormat('fr-FR', {
  dateStyle: 'long',
}).format(new Date('2025-03-14'));
// -> "14 mars 2025"

Always use Intl.NumberFormat and Intl.DateTimeFormat (or library equivalents). Never manually format numbers with string operations - decimal separators, grouping separators, and currency symbol positions vary by locale.

Configure RTL layout with CSS logical properties

/* Instead of physical directions: */
.card {
  margin-left: 16px;    /* DON'T */
  padding-right: 8px;   /* DON'T */
  text-align: left;     /* DON'T */
}

/* Use logical properties: */
.card {
  margin-inline-start: 16px;   /* DO */
  padding-inline-end: 8px;     /* DO */
  text-align: start;           /* DO */
}

Set the document direction with <html dir="rtl" lang="ar">. For bidirectional content, use the dir="auto" attribute on user-generated content containers.

See references/rtl-layout.md for the full migration guide.

Extract translatable strings from source code

For react-intl / FormatJS projects:

npx formatjs extract 'src/**/*.{ts,tsx}' --out-file lang/en.json --id-interpolation-pattern '[sha512:contenthash:base64:6]'

For i18next projects, use i18next-parser:

npx i18next-parser 'src/**/*.{js,jsx,ts,tsx}'

Run extraction in CI to catch untranslated strings before they reach production.

Handle missing translations with fallback chains

// i18next fallback chain
i18n.init({
  fallbackLng: {
    'pt-BR': ['pt', 'en'],
    'zh-Hant': ['zh-Hans', 'en'],
    default: ['en'],
  },
});

The fallback order should go: specific locale -> language family -> default language. Never show raw message keys (app.greeting.title) to users - always ensure the fallback chain terminates at a fully-translated locale.

Anti-patterns / common mistakes

Gotchas

1. i18next {{double braces}} vs ICU {single braces} silently produce different output - These two interpolation syntaxes are not interchangeable. Using ICU-style {name} in a project configured for i18next double-brace mode renders the literal string {name} to users. The i18next-icu plugin must be installed and configured before switching syntax. Always verify interpolation works end-to-end with a real locale before migrating your message catalog.

2. Intl.DateTimeFormat and Intl.NumberFormat constructors are expensive - cache them - Creating a new Intl.NumberFormat('de-DE', {...}) on every render or request has measurable overhead on high-traffic paths. Cache formatter instances keyed by locale and options string, or use a memoization wrapper. This is a common source of unexpected latency on locale-switching UIs.

3. Pluralization fallback to other masks missing plural forms in production - ICU's other is required and acts as a catch-all. If a translator omits the few form for Polish and a user has exactly 3 items, the other form renders silently - wrong grammar but no error. Run pluralization validation against CLDR's expected forms per locale in CI to catch missing categories before release.

4. RTL flipping CSS is not enough - icon direction and component orientation also need adjustment - Applying dir="rtl" flips text direction but does not automatically mirror directional icons (arrows, chevrons, back buttons), progress bars, or timeline components. Each directional UI element needs explicit RTL handling. Use a design system that supports dir-aware icon variants rather than flipping with CSS transforms.

5. String extraction tools miss dynamic message IDs - If your code constructs a message ID at runtime (t('error.' + code)), extraction tools like i18next-parser or formatjs extract cannot statically analyze it and will omit the key from the catalog. Translators never see it; users see raw keys. Use static string literals for all message IDs; handle dynamic content as parameters within a static message.

References

For detailed content on specific topics, read the relevant file from references/:

• references/icu-message-format.md - Full ICU syntax: plural, select, selectordinal, nested patterns, number/date skeletons
• references/pluralization.md - CLDR plural rules by language, plural categories, and common pitfalls
• references/rtl-layout.md - Complete RTL migration guide: CSS logical properties, bidirectional text, icon mirroring
• references/translation-workflows.md - TMS integration, XLIFF/JSON/PO formats, CI extraction, pseudolocalization

Only load a references file if the current task requires deep detail on that topic.

Companion check

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Skip entirely if recommended_skills is empty or all companions are already installed.