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 |
| Message extraction | Automated tooling scans source for translation keys | Prevents orphaned or missing translations |
| Pseudolocalization | Generate fake translations to test layout | Catches truncation, overflow, hardcoded strings |
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
Exploreagent to scan components for untranslated user-facing text - Set up full i18n pipeline: Use
Taskagent to configure locale detection, translation loading, and rendering - Plan localization architecture: Use
Planagent to design namespace structure, locale strategy, and translation workflow - Review translation coverage: Use
Taskagent to compare translation files and identify missing keys across locales - Code review for i18n compliance: Delegate to
code-revieweragent
If thetanstack-startskill is available, delegate server middleware and SSR locale patterns to it. If thetanstack-routerskill is available, delegate URL-based locale routing patterns to it. If thesveltekitskill is available, delegate SvelteKit-specific routing and hooks patterns to it. If theseoskill 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