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
- Check the Next.js version (
package.json) — must be 13+ with App Router - Check if i18n is already partially set up (look for
next-intl,next-i18next,[locale]routes) - Identify all pages/routes that need translation
- Identify all user-facing strings (hardcoded text in components)
- 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-intlStep 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]/:
- Create
src/app/[locale]/layout.tsxwith:
- 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
- Move existing pages into
src/app/[locale]/ - Each page should call
setRequestLocale(locale)for static generation
Step 7: Extract Strings into Translation Files
- Create
src/messages/en.jsonwith all user-facing strings organized by section:{"common": {"signIn": "Sign In",...}, "tools": {"tool-slug": {"title": "...", "description": "..."}}, "faq": {"tool-slug": [{"question": "...", "answer": "..."}]}} - Replace all hardcoded strings in components with
useTranslations():const t = useTranslations('common') return <button>{t('signIn')}</button> - 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:
- Launch one Codex task per locale (up to 7 in parallel) using
/codex-tasks - Each task reads
en.json, translates all strings, writes{locale}.json - 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
- 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
- Build the project:
npm run build— check that all static pages generate correctly - Test English URLs have no prefix:
https://example.com/tools - Test locale URLs have prefix:
https://example.com/es/tools - Verify sitemap has hreflang alternates
- Check RTL rendering for Arabic
- Run the translation verification script from Step 8
Common Pitfalls
public/sitemap.xmlconflicts with dynamicsrc/app/sitemap.tsin 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 continuitylocaleDetection: falseprevents 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