I18n configuration in next.config.js breaks app directory

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
      Platform: darwin
      Arch: x64
      Version: Darwin Kernel Version 22.6.0: Wed Jul  5 22:21:56 PDT 2023; root:xnu-8796.141.3~6/RELEASE_X86_64
    Binaries:
      Node: 20.5.0
      npm: 9.8.0
      Yarn: 3.5.0
      pnpm: 8.6.12
    Relevant Packages:
      next: 13.4.14-canary.0
      eslint-config-next: 13.4.13
      react: 18.2.0
      react-dom: 18.2.0
      typescript: 5.1.6
    Next.js Config:
      output: N/A

Which area(s) of Next.js are affected? (leave empty if unsure)

App Router, Internationalization (i18n), Routing (next/router, next/navigation, next/link)

Link to the code that reproduces this issue or a replay of the bug

https://github.com/fabio-nettis/next-i18n-error

To Reproduce

This issue is directly related to my previous issue #53665 You can find more about the discovery process of this bug in this discussion.

1. Clone the repository
2. Try to navigate to any of the defined pages
3. You will encounter a not found error for each of the defined routes.

Once you have seen the error you can then do the following steps to revert the broken app folder:

1. Remove i18n key from next config
2. Try to navigate to any of the defined pages
3. See the defined page

Describe the Bug

Since version 13.4.13-canary.0 an application previously working fine on version 13.4.12 returns a 404 status code for every page in the app folder when the i18n setting is set in the next config file.

Expected Behavior

Setting the i18n in the next config file should not break the whole application' routing.

Which browser are you using? (if relevant)

Chrome 114.0.5735.106 (Official Build) (x86_64)

How are you deploying your application? (if relevant)

next dev

_NEXT-1512

Clone the linked reproduction and run it with next dev, first with the i18n key in next.config.js and then with that key removed. Trace how the App Router handles the configuration and routes in the app folder. Done means the defined pages no longer return 404 responses when i18n is configured.