Use this skill when working with internationalization (i18n), localization (l10n), translation workflows, right-to-left (RTL) layout support, pluralization rules, or ICU MessageFormat syntax. Triggers on translating strings, setting up i18n libraries (react-intl, i18next, FormatJS), handling plural forms, formatting dates/numbers/currencies for locales, building translation pipelines, configuring RTL stylesheets, or writing ICU message patterns with select/plural/selectordinal.
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Use this skill when working with internationalization (i18n), localization (l10n), translation workflows, right-to-left (RTL) layout support, pluralization rules, or ICU MessageFormat syntax. Triggers on translating strings, setting up i18n libraries (react-intl, i18next, FormatJS), handling plural forms, formatting dates/numbers/currencies for locales, building translation pipelines, configuring RTL stylesheets, or writing ICU message patterns with select/plural/selectordinal.
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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
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.
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.
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.
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.
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.
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.
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.
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
Mistake
Why it's wrong
What to do instead
String concatenation for translations
Word order differs across languages; "Welcome to " + city fails in Japanese
Use ICU placeholders: "Welcome to {city}"
Hardcoded plural logic (n === 1)
Only works for English; breaks for Arabic (6 forms), Polish (4 forms), Russian (3 forms)
Use ICU {count, plural, ...} with CLDR rules
Using left/right CSS properties
Breaks RTL layouts for Arabic, Hebrew, Persian
Use CSS logical properties: inline-start/inline-end
Translating string fragments
"Click " + <Link>here</Link> + " to continue" is untranslatable as a whole
Use rich text formatting: "Click <link>here</link> to continue" with component interpolation
Embedding numbers in strings
"Page 1 of 5" via concatenation skips locale-aware number formatting
Use "Page {current} of {total}" with Intl.NumberFormat
Storing translations in code
Translations scattered across components make extraction and updates impossible
Centralize in JSON/XLIFF message catalogs, one file per locale
Assuming text length is constant
German is ~35% longer than English; UI clips or overflows
Design flexible layouts, test with pseudolocalization
Gotchas
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.
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.
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.
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.
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/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
On first activation of this skill in a conversation: check which companion skills are installed by running ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/ .claude/skills/ .agent/skills/ .agents/skills/ 2>/dev/null. Compare the results against the recommended_skills field in this file's frontmatter. For any that are missing, mention them once and offer to install:
