localization-engineer

Framework-agnostic internationalization (i18n) and localization (l10n) patterns for multilingual applications. Covers ICU message format, pluralization rules (CLDR), RTL layout with CSS logical properties, locale negotiation and detection strategies, SEO hreflang tags, cookie/header-based locale resolution, and translation workflow management. Use when adding multi-language support, choosing an i18n library, implementing pluralization or gender-aware messages, formatting dates/numbers/currencies by locale, handling RTL layouts, setting up locale detection, configuring hreflang SEO tags, or managing translation workflows.

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Repository: oakoss/agent-skills
Last source activity: February 23, 2026 at 04:27
Detected SKILL.md language: English
Stars: 14
Forks: 5

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name	localization-engineer
description	Framework-agnostic internationalization (i18n) and localization (l10n) patterns for multilingual applications. Covers ICU message format, pluralization rules (CLDR), RTL layout with CSS logical properties, locale negotiation and detection strategies, SEO hreflang tags, cookie/header-based locale resolution, and translation workflow management. Use when adding multi-language support, choosing an i18n library, implementing pluralization or gender-aware messages, formatting dates/numbers/currencies by locale, handling RTL layouts, setting up locale detection, configuring hreflang SEO tags, or managing translation workflows.
license	MIT
metadata	{"author":"oakoss","version":"2.0"}

Localization Engineer

Overview

Provides framework-agnostic patterns for building multilingual applications, covering the full i18n pipeline from locale detection through translation rendering. Addresses universal concerns like ICU message formatting, CLDR plural rules, RTL support, and SEO.

When to use: Adding multi-language support, locale-aware formatting, RTL layouts, pluralization, translation management, locale negotiation, multilingual SEO.

When NOT to use: Single-language apps with no internationalization plans, content that never changes locale, purely server-rendered static sites with no dynamic content.

Key decision: Choose Paraglide for compile-time type safety and minimal bundles in new projects. Choose i18next for broad ecosystem support and incremental adoption in existing projects. See the Library Selection table below.

Library Selection

Criteria	Paraglide JS	i18next
Architecture	Compile-time, tree-shakable functions	Runtime, plugin-based ecosystem
Type safety	Full (generated typed message functions)	Partial (requires manual type setup)
Bundle size	Minimal (only used messages shipped)	Larger runtime (~40 kB base + plugins)
Pluralization	Via message format plugins	Built-in with CLDR plural categories
Framework support	SvelteKit, TanStack Start, React Router, Astro, vanilla	React, Vue, Svelte, Angular, Node, vanilla
ICU support	Via `inlang-icu-messageformat-1` plugin	Via `i18next-icu` plugin
Translation tools	Fink editor, Sherlock VS Code extension	Locize, i18next-parser, many integrations
Locale strategies	Built-in (cookie, URL, header, custom)	Via `i18next-browser-languagedetector` plugin
SSR support	AsyncLocalStorage-based per-request	Separate i18next instance per request
Learning curve	Low (compiler generates simple functions)	Medium (large API surface, many plugins)
Namespace support	Flat message files per locale	Multi-namespace with lazy loading per route
Message format	Inlang format or ICU via plugin	i18next JSON v4 or ICU via plugin
Community size	Growing (inlang ecosystem)	Large (established since 2015)
Best for	New projects prioritizing bundle size and type safety	Existing projects needing broad ecosystem support

Quick Reference

Pattern	Approach	Key Points
ICU pluralization	`{count, plural, one {# item} other {# items}}`	CLDR categories: zero, one, two, few, many, other
ICU select (gender)	`{gender, select, male {He} female {She} other {They}}`	Covers gender-aware and variant-based messages
ICU selectordinal	`{rank, selectordinal, one {#st} two {#nd} other {#th}}`	Ordinal suffixes vary by language
Number formatting	`Intl.NumberFormat(locale, options)`	Currency, percent, unit formatting built into platform
Date formatting	`Intl.DateTimeFormat(locale, options)`	Avoid manual date string construction
Relative time	`Intl.RelativeTimeFormat(locale, options)`	"3 hours ago", "in 2 days" with locale-aware output
List formatting	`Intl.ListFormat(locale, { type: 'conjunction' })`	"Alice, Bob, and Charlie" with locale-aware conjunctions
RTL layout	CSS logical properties (`inline-start`, `inline-end`)	Replace `left`/`right` with logical equivalents
RTL detection	`<html dir="rtl" lang="ar">`	Set `dir` attribute based on locale
Locale from URL	`/en/about`, `/de/about`	Most SEO-friendly, clear to users
Locale from cookie	`locale=de` cookie	Persists preference across sessions
Locale from header	`Accept-Language: de-DE,de;q=0.9,en;q=0.8`	Browser preference, use as fallback
Hreflang tags	`<link rel="alternate" hreflang="de" href="...">`	One per locale plus `x-default`
Namespace splitting	Group translations by feature or route	Reduces bundle size via lazy loading

Common Mistakes

Mistake	Correct Pattern
Concatenating translated fragments into sentences	Use ICU message format with placeholders for proper grammar across locales
Using `margin-left`/`padding-right` with RTL	Use CSS logical properties (`margin-inline-start`, `padding-inline-end`)
Hardcoding plural rules (`count === 1`)	Use CLDR plural categories (some languages have zero, two, few, many forms)
Detecting locale only from IP geolocation	Use `Accept-Language` header, then cookie, then geolocation as last resort
Missing `x-default` in hreflang tags	Always include `<link rel="alternate" hreflang="x-default">` for fallback
One massive translation file per locale	Split into namespaces by feature for lazy loading and maintainability
Creating new i18next instance per component	Create once at app initialization, share across components
Formatting dates with string concatenation	Use `Intl.DateTimeFormat` for locale-aware date rendering
Storing locale in component state	Store in URL path or cookie for SSR compatibility and shareability
Using different i18n keys in client and server	Share translation files and key structure between client and server
Assuming all languages expand equally	Budget 30-40% extra space for German/Finnish translations vs English
Embedding text in images	Use CSS/SVG text overlays so translations can be applied dynamically
Using language codes without region variants	Use `en-US` vs `en-GB` when formatting differences matter (dates, currency)

Delegation

Audit codebase for hardcoded strings: Use Explore agent to scan components for untranslated user-facing text
Set up full i18n pipeline: Use Task agent to configure locale detection, translation loading, and rendering
Plan localization architecture: Use Plan agent to design namespace structure, locale strategy, and translation workflow
Review translation coverage: Use Task agent to compare translation files and identify missing keys across locales
Code review for i18n compliance: Delegate to code-reviewer agent

If the tanstack-start skill is available, delegate server middleware and SSR locale patterns to it. If the tanstack-router skill is available, delegate URL-based locale routing patterns to it. If the sveltekit skill is available, delegate SvelteKit-specific routing and hooks patterns to it. If the seo skill is available, delegate advanced hreflang and sitemap patterns to it.

References

Paraglide JS patterns -- compile-time i18n with typed message functions, SvelteKit/TanStack Start/React Router integration, locale strategies
i18next patterns -- runtime i18n with plugin ecosystem, React/Vue/Svelte/vanilla integration, namespaces, language detection
ICU message format -- pluralization, select, number/date formatting, nested messages, RTL support, hreflang SEO

localization-engineer

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More from this repository

More from this repository

Localization Engineer

Overview

Library Selection

Quick Reference

Common Mistakes

Delegation

References

Localization Engineer

Overview

Library Selection

Quick Reference

Common Mistakes

Delegation

References