When visiting a non-locale prefixed path (`/hello` instead of `/fr/hello`) we don't trigger locale redirects currently so if another locale is matched we need to ensure this is reset to the `defaultLocale` for rendering to prevent a mis-match on the client and the server.
This also fixes hydration errors from occurring with `asPath` for `getServerSideProps` pages due to `normalizeLocalePath` expecting only a pathname and `asPath` containing `hash` and `query values also.
Fixes: https://github.com/vercel/next.js/issues/18337
Fixes: https://github.com/vercel/next.js/issues/18510
This adds the initial changes outlined in the [i18n routing RFC](https://github.com/vercel/next.js/discussions/17078). This currently treats the locale prefix on routes similar to how the basePath is treated in that the config doesn't require any changes to your pages directory and is automatically stripped/added based on the detected locale that should be used.
Currently redirecting occurs on the `/` route if a locale is detected regardless of if an optional catch-all route would match the `/` route or not we may want to investigate whether we want to disable this redirection automatically if an `/index.js` file isn't present at root of the pages directory.
TODO:
- [x] ensure locale detection/populating works in serverless mode correctly
- [x] add tests for locale handling in different modes, fallback/getStaticProps/getServerSideProps
To be continued in fall-up PRs
- [ ] add tests for revalidate, auto-export, basePath + i18n
- [ ] add mapping of domains with locales
- [ ] investigate detecting locale against non-index routes and populating the locale in a cookie
x-ref: https://github.com/vercel/next.js/issues/17110
