output: "standalone" + next-i18next: user config not copied into standalone output on 16.3.x (regression from 16.2.x)

Link to the code that reproduces this issue

https://github.com/nitipat009/next-standalone-i18next-repro

To Reproduce

1. git clone https://github.com/nitipat009/next-standalone-i18next-repro
2. cd next-standalone-i18next-repro && npm install
3. npm run build
4. ls .next/standalone/next-i18next.config.js — file is missing
5. node .next/standalone/server.js then curl http://localhost:3000/ — returns 500

Current vs. Expected behavior

With output: "standalone" and next-i18next's standard setup (appWithTranslation in _app, serverSideTranslations in a page's getServerSideProps), I expected next-i18next.config.js to be copied into .next/standalone/ as it is on next@16.2.12. Instead, on next@16.3.0 and later it is missing, and the standalone server throws Error: next-i18next was unable to find a user config at /app/next-i18next.config.js on any request.

Bisection (change the next version in package.json of the repro repo and repeat the steps above):

Build uses next build --webpack (Turbopack is not involved).

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0
Binaries:
  Node: 22.15.0
  npm: 10.9.2
Relevant Packages:
  next: 16.3.0 (also reproduces on 16.3.1-16.3.5)
  next-i18next: 15.4.3
  react: 19.2.1
  react-dom: 19.2.1
Next.js Config:
  output: standalone

Which area(s) are affected?

Output (export/standalone), Webpack, Internationalization (i18n)

Which stage(s) are affected?

next build (local), next start (local)

Additional context

Follow-up to #98626 and #98625, both auto-closed for a missing/invalid reproduction link — apologies, the link is now the sole content of the "Link to the code that reproduces this issue" field.

I am hosting via Docker with output: "standalone", but the issue reproduces the same way with a plain node .next/standalone/server.js locally, no Docker required.

Start with the linked next-standalone-i18next-repro repository and compare the standalone output from next@16.2.12 with 16.3.0 using npm run build. Inspect how the webpack standalone build handles next-i18next.config.js, then verify that .next/standalone/next-i18next.config.js is present and that node .next/standalone/server.js followed by curl returns 200.