Next.js Internationalized Routing not working

First of all, this effort is fantastic, thank you for pursuing it.

One of the Next.js features that require a Next.js server is [Internationalized Routing](https://nextjs.org/docs/advanced-features/i18n-routing). In a demo project I created, configuring Next.js internationalization for sub-path routing (cf. https://nextjs.org/docs/advanced-features/i18n-routing#sub-path-routing) works exactly as expected when running the page locally using `firebase serve`. However, the deployed version only always uses the defined `defaultLocale`.

Notably, the other language version of the page is available at the sub path (in my case, `/de`) after deployment - but server-side forwarding does not occurr as expected. Possibly the `Accept-Language` header is not passed to the SSR Cloud Function correctly in the deployed version?