next-i18n-router automatically changing the locale variable to a random locale variable sometimes

I have used three packages for implementing i18n on a website.

"next-i18n-router": "^5.4.0",
"i18next-resources-to-backend": "^1.2.0",
"react-i18next": "^14.1.0",

Here is my i18nConfig:

{
locales: locales,
defaultLocale: "en", 
prefixDefault: false, 
}

The problem is, the cookie variable "NEXT_LOCALE" changes automatically sometimes even if I try to access any route with the defaultLocale. Let's say I try to access mywebsite.com/products, for some unknown reasons, I will be redirected to mywebsite.com/de/products.

I've tried setting the value of the "NEXT_LOCALE" to "en" too but that din't work. I also created a client component that checks the cookie and if "NEXT_LOCALE" cookie does not exist, the component will set the cookie with value "en" which din't help either. How can I fix this issue?

You can try using different packages. For my case, I've used "negotiator" and "@formatjs/intl-localematcher".

Here is a simple middleware with those two packages for detecting locale from the url parameter:

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
}

export function middleware(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 const config = {
  // Matcher ignoring `/_next/` and `/api/`
  matcher: ['/((?!api|_next/static|_next/image|favicon.ico).*)']
}