I18n skill

Internationalize a Next.js Project

Add complete internationalization to a Next.js (App Router) project using next-intl v4. This skill handles routing, translation files, sitemap hreflang, and bulk translation across all locales.

Step 1: Assess the Project

1. Check the Next.js version (package.json) — must be 13+ with App Router
2. Check if i18n is already partially set up (look for next-intl, next-i18next, [locale] routes)
3. Identify all pages/routes that need translation
4. Identify all user-facing strings (hardcoded text in components)
5. Ask the user which locales to support (default recommendation: en, es, fr, de, pt, ja, ar, zh, zh-tw, id, vi, ms, ru, hi)

Step 2: Install Dependencies

npm install next-intl

Step 3: Create i18n Configuration Files

Create 4 files under src/i18n/:

src/i18n/config.ts

export const locales = ['en', 'es', 'fr', 'de', 'pt', 'ja', 'ar', 'zh', 'zh-tw', 'id', 'vi', 'ms', 'ru', 'hi'] as const

export type Locale = (typeof locales)[number]
export const defaultLocale: Locale = 'en'

export const localeNames: Record<Locale, string> = {
  en: 'English',
  es: 'Espanol',
  fr: 'Francais',
  de: 'Deutsch',
  pt: 'Portugues',
  ja: '日本語',
  ar: 'العربية',
  zh: '简体中文',
  'zh-tw': '繁體中文',
  id: 'Bahasa Indonesia',
  vi: 'Tieng Viet',
  ms: 'Bahasa Melayu',
  ru: 'Русский',
  hi: 'हिन्दी',
}

export const rtlLocales: Locale[] = ['ar']

src/i18n/routing.ts

import { defineRouting } from 'next-intl/routing'
import { defaultLocale, locales } from './config'

export const routing = defineRouting({
  locales,
  defaultLocale,
  localePrefix: 'as-needed', // English URLs stay clean, other locales get /es/, /fr/, etc.
})

src/i18n/navigation.ts

import { createNavigation } from 'next-intl/navigation'
import { routing } from './routing'

export const { Link, redirect, usePathname, useRouter } = createNavigation(routing)

src/i18n/request.ts

import { getRequestConfig } from 'next-intl/server'
import { routing } from './routing'

export default getRequestConfig(async ({ requestLocale }) => {
  let locale = await requestLocale
  if (!locale || !routing.locales.includes(locale as any)) {
    locale = routing.defaultLocale
  }
  return {
    locale,
    messages: (await import(`../messages/${locale}.json`)).default,
  }
})

Step 4: Create Middleware

Create src/middleware.ts:

import createMiddleware from 'next-intl/middleware'
import { routing } from '@/i18n/routing'

export default createMiddleware({
  ...routing,
  localeDetection: false, // Don't auto-redirect based on Accept-Language
})

export const config = {
  matcher: ['/((?!_next|api|images|fonts|favicon|sitemap|robots).*)'],
}

Key decision: localeDetection: false prevents auto-redirecting users based on browser language. This keeps English URLs stable for SEO. Users can manually switch languages via a language selector.

Step 5: Update next.config

Wrap the existing config with createNextIntlPlugin:

import createNextIntlPlugin from 'next-intl/plugin'
const withNextIntl = createNextIntlPlugin('./src/i18n/request.ts')

// ... existing config ...
export default withNextIntl(nextConfig)

Step 6: Add [locale] Dynamic Route

Move all page content under src/app/[locale]/:

1. Create src/app/[locale]/layout.tsx with:

   • generateStaticParams() returning all locales
   • setRequestLocale(locale) call
   • <NextIntlClientProvider> wrapping children
   • <html lang={locale} dir={rtlLocales.includes(locale) ? 'rtl' : 'ltr'}>
   • Hreflang <link> tags in <head> for all locales + x-default

2. Move existing pages into src/app/[locale]/

3. Each page should call setRequestLocale(locale) for static generation

Step 7: Extract Strings into Translation Files

1. Create src/messages/en.json with all user-facing strings organized by section:

   {
     "common": { "signIn": "Sign In", ... },
     "tools": { "tool-slug": { "title": "...", "description": "..." } },
     "faq": { "tool-slug": [{ "question": "...", "answer": "..." }] }
   }
   

2. Replace all hardcoded strings in components with useTranslations():

   const t = useTranslations('common')
   return <button>{t('signIn')}</button>
   

3. For server components, use getTranslations():

   const t = await getTranslations('common')
   

Step 8: Translate to All Locales

For each non-English locale, create src/messages/{locale}.json with the same structure as en.json.

Translation Strategy

Use parallel Codex agents via the codex-tasks skill to save Claude credits:

1. Launch one Codex task per locale (up to 7 in parallel) using /codex-tasks
2. Each task reads en.json, translates all strings, writes {locale}.json
3. Codex prompt should include:
   • The full en.json content (or path to read it)
   • Target language name and locale code
   • Instructions:
     • Translate naturally, not literally
     • Keep technical terms in English (PowerPoint, PDF, API, etc.)
     • Preserve JSON structure exactly (same keys, same nesting)
     • Preserve interpolation variables like {count}, {name} unchanged
     • Write the result to src/messages/{locale}.json
4. After Codex tasks complete, verify the results using the verification script below — Codex output quality varies and must be checked

Verification

After translation, run a verification script to catch issues:

import json

locales = ['es', 'fr', 'de', 'pt', 'ja', 'ar', 'zh', 'zh-tw', 'id', 'vi', 'ms', 'ru', 'hi']
english_words = ['the ', 'and ', 'you ', 'your ', 'our ', 'this ', 'that ', 'with ', 'from ', 'will ']

with open('src/messages/en.json') as f:
    en = json.load(f)

for loc in locales:
    with open(f'src/messages/{loc}.json') as f:
        data = json.load(f)

    # Check: missing sections
    missing = [s for s in en if s not in data]

    # Check: residual English content
    eng_count = 0
    def check(d):
        nonlocal eng_count  # won't work in inline script; use list trick
        if isinstance(d, dict):
            for v in d.values(): check(v)
        elif isinstance(d, str):
            if sum(1 for w in english_words if w in d.lower()) >= 3:
                eng_count += 1
    check(data)

    status = 'OK' if not missing and eng_count == 0 else 'ISSUES'
    print(f'{loc}: {status} (missing={len(missing)}, english={eng_count})')

Step 9: Update Sitemap with Hreflang

Update src/app/sitemap.ts to include hreflang alternates:

import { MetadataRoute } from 'next'
import { locales } from '@/i18n/config'

const baseUrl = 'https://www.example.com'

function buildAlternates(path: string): Record<string, string> {
  const alternates: Record<string, string> = {}
  for (const locale of locales) {
    const prefix = locale === 'en' ? '' : `/${locale}`
    alternates[locale] = `${baseUrl}${prefix}${path}`
  }
  return alternates
}

export default function sitemap(): MetadataRoute.Sitemap {
  return pages.map((path) => ({
    url: `${baseUrl}${path}`,
    lastModified: new Date(),
    alternates: { languages: buildAlternates(path) },
  }))
}

Important: Generate one canonical URL per page with hreflang alternates, NOT one URL per locale. This prevents duplicate content in search results.

Step 10: Add Language Selector (Optional)

Add a language switcher component that uses useRouter and usePathname from @/i18n/navigation to switch locales while preserving the current path.

Step 11: Verify

1. Build the project: npm run build — check that all static pages generate correctly
2. Test English URLs have no prefix: https://example.com/tools
3. Test locale URLs have prefix: https://example.com/es/tools
4. Verify sitemap has hreflang alternates
5. Check RTL rendering for Arabic
6. Run the translation verification script from Step 8

Common Pitfalls

• public/sitemap.xml conflicts with dynamic src/app/sitemap.ts in dev mode — delete the static one or rename it
• Middleware matcher must exclude _next, api, sitemap, robots, and static asset paths
• localePrefix: 'as-needed' is critical — it keeps default locale URLs clean for SEO continuity
• localeDetection: false prevents unwanted redirects that break SEO and confuse users
• Large translation files (5000+ lines per locale) can make git pushes fail — use git config http.postBuffer 524288000
• Verify translations thoroughly — automated translation often produces mixed-language output; always verify with the English word detection script after Codex tasks complete

Locale Count Reference

• 14 locales x N pages = 14N static pages at build time
• Each locale JSON file is typically 2-5x the size of en.json (CJK characters, verbose languages)
• Build time increases linearly with locale count