Anyone...

Next Auth provides a withAuth function which allows you to wrap an existing middleware inside.
Hence, try the following:

import { withAuth } from "next-auth/middleware"
const middleware = withAuth((req) => ...)
export default middleware;

Next.js doesn't support multiple middlewares, you must combine them into a middleware in this case

Okay got it but what about Matcher???
I got negative loop regex for i18n and for next-auth just protected routes array...

import {withAuth} from "next-auth/middleware"
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

import { i18n } from '@/i18n.config'

import { match as matchLocale } from '@formatjs/intl-localematcher'
import Negotiator from 'negotiator'

function getLocale(request: NextRequest): string | undefined {
  const negotiatorHeaders: Record<string, string> = {}
  request.headers.forEach((value, key) => (negotiatorHeaders[key] = value))

  // @ts-ignore locales are readonly
  const locales: string[] = i18n.locales
  const languages = new Negotiator({ headers: negotiatorHeaders }).languages()

  const locale = matchLocale(languages, locales, i18n.defaultLocale)
  return locale
}

const middleware = withAuth((request: NextRequest)=> {
  const pathname = request.nextUrl.pathname
  const pathnameIsMissingLocale = i18n.locales.every(
    locale => !pathname.startsWith(`/${locale}/`) && pathname !== `/${locale}`
  )

  // Redirect if there is no locale
  if (pathnameIsMissingLocale) {
    const locale = getLocale(request)
    return NextResponse.redirect(
      new URL(
        `/${locale}${pathname.startsWith('/') ? '' : '/'}${pathname}`,
        request.url
      )
    )
  }
})
export default middleware;

what about this

for i18n:
export const config = {
  // Matcher ignoring `/_next/` and `/api/`
  matcher: ['/((?!api|_next/static|_next/image|images).*)']
}
for next-auth:
export const config = { matcher: ["/dashboard"] }

@fuma 💙 joulev ?

You may combine the matcher if they both share the same routes, but seems in your case, you only wanna protect the dashboard page.

Actually withAuth also offers a simple method to achieve it, you can use withAuth in this way:

if (request.nextUrl.pathname === "/dashboard")
    return withAuth(request)

so that instead of wrapping your middleware inside of withAuth, you can use it as a function

Check its type definitions for other usages