<objective> Implements internationalization for Next.js 16 App Router apps via two approaches: next-intl (recommended — full Server Components support, type-safe messages, ICU MessageFormat, async message loading, proxy.ts-compatible routing) or a lightweight DIY dictionary approach using dynamic imports.

Covers [locale]-prefixed routing, localized navigation components (Link, redirect, usePathname, useRouter), date/number/currency/relative-time formatting, RTL language support, SEO hreflang tags, and localized special files ([locale]/error.tsx, [locale]/not-found.tsx, global-error.tsx). This is the Next.js-specific i18n skill (App Router routing, proxy.ts integration) — for plain React apps without Next.js see react-i18n, and for Astro see astro-i18n. </objective>

Next.js 16 Internationalization

Complete i18n solution with next-intl or DIY dictionary approach.

Agent Workflow (MANDATORY)

Before ANY implementation, spawn 3 agents in parallel, one Agent call each with a name:

fuse-ai-pilot:explore-codebase - Analyze existing i18n setup and message files
fuse-ai-pilot:research-expert - Verify latest next-intl docs via Context7/Exa
mcp__context7__query-docs - Check locale config and patterns

After implementation, run fuse-ai-pilot:sniper for validation.

Overview

When to Use

Building multilingual Next.js 16 applications
Need locale-based routing with [locale] dynamic segment
Implementing language switcher and URL localization
Formatting dates, numbers, currencies, and relative times per locale
SEO optimization with hreflang tags and localized metadata
Supporting right-to-left (RTL) languages

Why next-intl

| Feature | Benefit | |---------|---------| | App Router native | Full Server Components support | | Type-safe messages | TypeScript autocompletion for keys | | ICU MessageFormat | Pluralization, gender, select expressions | | Async message loading | Load translations on-demand per locale | | proxy.ts compatible | Works with Next.js 16 proxy pattern | | Rich formatting | Dates, numbers, lists, relative time |

Two Approaches

1. next-intl (Recommended)

Full-featured library with routing, formatting, and type safety. Best for production applications needing comprehensive i18n support.

2. DIY Dictionary

Lightweight approach using dynamic imports for simple translation needs. Good for projects wanting minimal dependencies.

SOLID Architecture

Module Structure

All i18n code organized in modules/cores/i18n/:

config/ - Routing configuration, locale definitions
interfaces/ - TypeScript types for messages and locales
services/ - Request handlers, message loaders
messages/ - JSON translation files per locale

File Locations

src/modules/cores/i18n/src/config/routing.ts - Locale routing config
src/modules/cores/i18n/messages/en.json - English translations
src/modules/cores/i18n/messages/es.json - Spanish translations
proxy.ts - Locale detection and redirect logic

Routing Patterns

Locale Segment

All routes prefixed with [locale] dynamic segment:

/en/about → English about page
/es/about → Spanish about page
/ → Redirects to default locale

Navigation Components

Use localized navigation from next-intl for automatic locale handling:

Link - Locale-aware anchor links
redirect - Server-side locale redirect
usePathname - Current path without locale
useRouter - Programmatic navigation

Reference Guide

| Need | Reference | |------|-----------| | Initial setup | installation.md, routing-setup.md | | Route config | routing-config.md, middleware-proxy.md | | Translations | translations.md, messages-validation.md | | Formatting | formatting.md | | Components | server-components.md, client-components.md | | Navigation | navigation.md | | TypeScript | typescript.md | | SEO | seo.md | | Testing | testing.md | | DIY approach | diy-dictionaries.md, diy-locale-detection.md |

Message Formatting

ICU MessageFormat

Pluralization - {count, plural, one {# item} other {# items}}
Select - {gender, select, male {He} female {She} other {They}}
Rich text - Support for bold, italic, links in messages

Formatters

formatDate - Locale-aware date formatting
formatNumber - Currency, percentages, decimals
formatList - Conjunction/disjunction lists
formatRelativeTime - "2 hours ago", "in 3 days"

Best Practices

Type-safe keys - Use TypeScript for message key autocompletion
Namespace messages - Organize by feature/page for maintainability
Server-first - Load translations on server, avoid client bundles
SEO hreflang - Add alternate links for all locales
RTL support - Use dir attribute for right-to-left languages
Fallback locale - Configure default for missing translations

Error Handling

Special Files

Localized error and loading states require specific handling:

[locale]/error.tsx - Localized error boundary
[locale]/not-found.tsx - Localized 404 page
global-error.tsx - Root error fallback

See error-files.md for complete patterns.