Software Localisation Skill

Software Localisation

Use this skill for production web-app i18n and l10n: library choice, message catalogs, ICU usage, locale routing, translation workflow, RTL, and release gates. The goal is not just translated strings. The goal is locale-safe product behavior.

Quick Reference

| Need | Starting Direction | |------|--------------------| | React or general flexibility | i18next / react-i18next | | ICU-first catalogs | react-intl / FormatJS | | Vue | vue-i18n | | Angular | @angular/localize | | Next.js App Router | next-intl | | smaller bundle bias | Lingui | | stronger generated wrappers | typesafe-i18n only when deliberate |

When to Use This Skill

setting up or debugging i18n in React, Vue, Angular, or Next.js
choosing libraries and catalog strategy
implementing ICU pluralisation, formatting, and locale detection
adding RTL support
configuring extraction, translation workflow, or TMS integration
fixing missing translations, mixed-language regressions, or bad locale fallback behavior

Route Elsewhere

general frontend architecture -> software-frontend
international SEO and hreflang -> marketing-seo
cross-cultural UX and market adaptation -> software-ui-ux-design or marketing-geo-localization
general WCAG/ARIA compliance and European Accessibility Act (EAA) readiness -> software-accessibility; this skill only covers the i18n-specific slice (lang/dir propagation, script-aware line height, IME) in references/accessibility-i18n.md

When Not to Use This Skill

The product has one locale and no committed plan to add more — don't pre-build ICU catalogs, TMS integration, or locale routing "just in case." Ship plain strings and revisit when a second locale is real.
The ask is "translate this text" with no code, catalog, or product surface involved — that's a translation task, not a localisation-engineering task; do it directly.
The ask is about international SEO structure (hreflang, ccTLD vs subfolder) with no i18n implementation involved -> route to marketing-seo instead.

Workflow

Confirm framework, locale count, route strategy, and translation workflow.
Choose the library and catalog model.
Define explicit-route resolution separately from first-entry locale negotiation and persistence.
Implement ICU or equivalent message formatting correctly.
Add extraction, review, and missing-key controls.
Add RTL and visual regression coverage where needed.
Block release on mixed-language or unsafe fallback behavior for indexable or customer-visible routes.

ASCII Flow

Localisation task
  -> Identify platform, source-of-truth catalog, and affected locales
  -> Choose key, ICU, formatting, and fallback strategy
  -> Patch durable source, not only generated output
  -> Check missing keys, plural rules, RTL, and text expansion
  -> Run generation or validation scripts
  -> Report locale coverage and residual translation risk

Library Selection Rules

| Situation | Library | Why | |---|---|---| | React / TypeScript, general flexibility | i18next + react-i18next | Best selector API and plugin ecosystem | | ICU-first catalogs, FormatJS tooling already in use | react-intl | Tightest ICU integration; FormatJS extract/compile pipeline | | Next.js App Router | next-intl | Built for RSC + App Router; automatic locale routing | | Vue | vue-i18n | Framework-native; best Vue tooling integration | | Angular | @angular/localize | Build-time extraction and AOT compilation | | Smaller bundle budget | Lingui | Smallest runtime; macro-based message extraction | | Team explicitly wants generated type wrappers | typesafe-i18n | Full key-type safety; high maintenance model |

Do not choose by popularity alone. Choose by routing model, extraction needs, ICU expectations, and team maintenance habits.

ICU MessageFormat 2 (MF2): Not Yet a Default Choice

MF2 is standardized at the syntax level in Unicode's LDML spec (stabilized through LDML 47-48), which makes it tempting to treat as "the new ICU." Do not migrate production catalogs to it as of mid-2026: ICU's own reference implementations are still draft/technology-preview status, and none of react-intl/FormatJS, i18next, vue-i18n, or Lingui has shipped a production MF2 migration path, and no mainstream TMS round-trips it. Keep using MessageFormat 1 / ICU syntax (documented in references/icu-message-format.md) and re-check adoption status before recommending a switch — this is a common "the spec is final, so it must be safe to use" misdiagnosis.

Core Rules

Encoding and content model

use UTF-8 end to end
never concatenate translatable strings
use interpolation and ICU plural or select rules instead of ad hoc formatting

Locale routing and fallback

an explicit locale in a valid URL or deep link wins for that request; preserve it through navigation and canonical metadata
when no explicit locale is present, negotiate from authenticated preference, persisted choice, supported Accept-Language, then default locale
a stored preference may redirect an unlocalized entry route, but must not silently rewrite a shared explicit-locale URL
always define a fallback locale
never silently fall back to English on indexable non-English routes
metadata, breadcrumbs, JSON-LD, and visible copy must stay in the same locale

Translation workflow

extract keys, do not hand-copy them
keep namespace structure stable
add translator context, glossary rules, and review gates
hardcoded string detection and missing-key checks should run in CI

RTL and accessibility

use CSS logical properties
set dir="rtl" where required
test with real RTL content
verify BiDi handling, icons, and screen-reader behavior across locales

Production Gates

| Gate | Failure condition | Remediation | |---|---|---| | Mixed-language output | Any locale-routed page renders keys from a different locale | Missing-key CI check catches before merge | | Missing-key bleed | Core UX or marketing route shows a key ID or raw fallback string | Extraction + catalog diff in CI pipeline | | Machine translation on release path | MT output inserted without glossary, tone, or reviewer gate | Add human review step for all customer-visible locales | | Locale switch drops state or breaks navigation | User changes locale and loses cart, form, or route state | Centralize locale state; separate from route/cookie | | RTL launched without visual validation | Arabic/Hebrew/Farsi layout broken on launch | Require visual pass on one RTL locale before release | | EU-market product ships accessible markup but only one language | Passes WCAG in English yet still fails the combined EAA + consumer-language bar in non-English EU markets | Review accessibility and localisation together for EU-facing surfaces; see references/accessibility-i18n.md |

Known Traps

| Trap | Prevention | |---|---| | Keys drift between extraction, TMS, and runtime — fallback "works" but locale is broken | Run missing-key checks in CI; diff extraction output against TMS catalog before release | | Visible strings translated, but validation messages / metadata / emails / JSON-LD left in English | Enumerate all locale surfaces (UI, email, SEO, legal) at project start; treat each as a separate test gate | | String concatenation for grammar-sensitive or gendered copy | Use ICU {count, plural, ...} / {gender, select, ...}; never "Hello " + name | | Only Latin-script locales tested | Require one long-string locale (de/ru) and one non-Latin (ja/ar) before "complete" | | Locale persisted separately in route, cookie, and client state with no precedence rule | Separate resolution rules: explicit URL/deep-link locale for the request; otherwise authenticated preference > persisted choice > supported Accept-Language > default | | Machine translation shipped without glossary or review | Require glossary, tone rules, and human review gate for all customer-visible content | | Plural category count assumed from memory (e.g. "French is just one/other like English") | CLDR revises per-language category counts over time (French now has one, many, other); verify against the current CLDR plural rules chart, don't hardcode from a prior project | | Translated content rendered as raw HTML (v-html, dangerouslySetInnerHTML, ICU HTML tags) with no CSP/Trusted Types | Real XSS advisories exist for this exact pattern in both vue-i18n (CVE-2025-53892) and Angular's i18n pipeline (CVE-2026-27970) — treat translation-file write access as privileged and pin patched library versions |

Common Anti-Patterns

| Anti-pattern | Correct approach | |---|---| | English as silent fallback on locale-routed pages | Fail visibly in CI or preview when a key is missing in a non-default locale | | Business logic encoded in translation keys | Keys represent UI text; business logic belongs in code | | Translations split by developer convenience | Split by stable product domain and runtime loading boundary | | CSS directional properties (left, right) patched per-view for RTL | Use CSS logical properties (inline-start, inline-end) from the start | | Each product surface (marketing, support, product) runs separate locale logic | Centralize fallback, formatting, and detection in one shared locale layer |

Ops Runbook

For large locale catalogs or mixed-language incidents:

diff keys first
translation pass second
treat marketing and SEO locale gaps as release blockers
chunk large locale files instead of reading them in one pass

Use references/ops-runbook.md for the detailed triage procedure.

Navigation

Core references

Templates and data

Maintenance

python3 scripts/check_urls.py
python3 scripts/check_examples.py

Related Skills

Fact-Checking

Known bugs, regressions, framework/compiler/runtime footguns, and version-specific crash or workaround guidance must be verified against current primary web sources before being treated as current fact.
Verify current library support, framework compatibility, and recommended tooling before final answers.
Prefer official docs, package registries, and release notes for version-sensitive guidance.
If web access is unavailable, mark version and maintenance guidance as unverified.

Learnings Loop

When prior decisions or pitfalls are relevant, consult learnings.consolidated.md if present; use learnings.md only for needed history or as the available fallback. Otherwise skip both.

After applying it, if you encountered a pattern worth remembering, a mistake worth preventing, or a domain fact that surprised you, append one dated bullet to learnings.md via agents-skills-feedback-loop/scripts/append_learning.py. Do not modify SKILL.md itself.

Agent Skills: Software Localisation

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