Software Localisation

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

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

1. Confirm framework, locale count, route strategy, and translation workflow.
2. Choose the library and catalog model.
3. Define locale detection and persistence order.
4. Implement ICU or equivalent message formatting correctly.
5. Add extraction, review, and missing-key controls.
6. Add RTL and visual regression coverage where needed.
7. 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

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

• locale selection should prioritize user preference, then route or URL, then cookie, then headers, then default locale
• 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

Known Traps

Common Anti-Patterns

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

• references/framework-guides.md
• references/icu-message-format.md
• references/translation-workflows.md
• references/rtl-support.md
• references/locale-handling.md
• references/testing-i18n.md
• references/accessibility-i18n.md
• references/content-management-patterns.md
• references/ops-runbook.md

Templates and data

• assets/react-i18next-setup.md
• assets/vue-i18n-setup.md
• assets/nextjs-i18n-setup.md
• data/sources.json

Maintenance

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

Related Skills

• software-frontend
• marketing-seo
• software-accessibility

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

Before applying this skill on a non-trivial task, read learnings.consolidated.md in this directory (and learnings.md if present).

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.