Function detectLocale

packages/next-routing/src/i18n.ts:196–269

(params: {
  pathname: string
  hostname: string | undefined
  cookieHeader: string | undefined
  acceptLanguageHeader: string | undefined
  i18n: I18nConfig
})

 * Detects the appropriate locale based on path, domain, cookie, and accept-language
 */
export function detectLocale(params: {
  pathname: string
  hostname: string | undefined
  cookieHeader: string | undefined
  acceptLanguageHeader: string | undefined
  i18n: I18nConfig
}): {
  locale: string
  pathnameWithoutLocale: string
  localeInPath: boolean
} {
  const { pathname, hostname, cookieHeader, acceptLanguageHeader, i18n } =
    params

  // 1. Check if locale is in the pathname
  const pathLocaleResult = normalizeLocalePath(pathname, i18n.locales)
  if (pathLocaleResult.detectedLocale) {
    return {
      locale: pathLocaleResult.detectedLocale,
      pathnameWithoutLocale: pathLocaleResult.pathname,
      localeInPath: true,
    }
  }

  // If locale detection is disabled, use domain locale or default locale
  if (i18n.localeDetection === false) {
    const domainLocale = detectDomainLocale(i18n.domains, hostname)
    return {
      locale: domainLocale?.defaultLocale || i18n.defaultLocale,
      pathnameWithoutLocale: pathname,
      localeInPath: false,
    }
  }

  // 2. Check cookie (priority over domain when locale detection is enabled)
  const cookieLocale = getCookieLocale(cookieHeader, i18n.locales)
  if (cookieLocale) {
    return {
      locale: cookieLocale,
      pathnameWithoutLocale: pathname,
      localeInPath: false,
    }
  }

  // 3. Check accept-language header (priority over domain when locale detection is enabled)
  const acceptLocale = getAcceptLanguageLocale(
    acceptLanguageHeader || '',
    i18n.locales
  )
  if (acceptLocale) {
    return {
      locale: acceptLocale,
      pathnameWithoutLocale: pathname,
      localeInPath: false,
    }
  }

  // 4. Check domain locale