fix i18n data pathname resolving (#68947)

### What
When using middleware + i18n in pages router, and upon navigation to a
dynamic route, the wrong locale would be served in `getServerSideProps`.

### Why
The route resolver code handles detecting the initial locale by
splitting the path and looking at the first non-empty index to determine
which locale was set. However, it does this assuming it has a regular
path name, and not a `/_next/data` URL.

When middleware is present, the route resolver code hits the
`middleware_next_data` branch, which doesn't have any logic to properly
set the locale. This means that resolveRoutes will return the default
locale rather than the locale from the path.

In the non-middleware case, this would normally flow through
`handleNextDataRequest` in base-server which has handling to infer i18n
via `i18nProvider`. This branch is functionally very similar to what
we're doing in `resolveRoutes` but it does so in a different way: it
reconstructs the URL without the `/_next/data` information and then
attaches locale information. However because `middleware_next_data`
rewrites the pathname to the actual route (ie
`/_next/data/development/foo.json` -> `/foo`), `handleNextDataRequest`
won't handle that request since it's no longer a data request.

It's strange to me that `handleNextDataRequest` and
`middleware_next_data` are doing similar path normalization in
completely different ways, but that was a deeper rabbit hole and simply
removing the normalization logic in `resolveRoutes` causes other
problems.

### How
Since data requests that flow through this middleware matcher logic
aren't going to be handled by `handleNextDataRequest`, I've updated
`middleware_next_data` to perform the logic of attaching the locale
information to the query. Initially I was going to do this for anything
that calls `normalizeLocalePath` but it seems like other branches of
`resolveRoutes` do it in the matcher function directly.

Fixes #54217
Closes NDX-116

packages/next/src/server/lib/router-utils/resolve-routes.ts

@@ -391,6 +391,17 @@ export function getResolveRoutes(
               normalized = normalizers.postponed.normalize(normalized, true)
             }
 
+            if (config.i18n) {
+              const curLocaleResult = normalizeLocalePath(
+                normalized,
+                config.i18n.locales
+              )
+
+              if (curLocaleResult.detectedLocale) {
+                parsedUrl.query.__nextLocale = curLocaleResult.detectedLocale
+              }
+            }
+
             // If we updated the pathname, and it had a base path, re-add the
             // base path.
             if (updated) {

test/e2e/i18n-navigations-middleware/i18n-navigations-middleware.test.ts

@@ -0,0 +1,43 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('i18n-navigations-middleware', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should respect selected locale when navigating to a dynamic route', async () => {
+    const browser = await next.browser('/')
+    // change to "de" locale
+    await browser.elementByCss("[href='/de']").click()
+    const dynamicLink = await browser.waitForElementByCss(
+      "[href='/de/dynamic/1']"
+    )
+    expect(await browser.elementById('current-locale').text()).toBe(
+      'Current locale: de'
+    )
+
+    // navigate to dynamic route
+    await dynamicLink.click()
+
+    // the locale should still be "de"
+    expect(await browser.elementById('dynamic-locale').text()).toBe(
+      'Locale: de'
+    )
+  })
+
+  it('should respect selected locale when navigating to a static route', async () => {
+    const browser = await next.browser('/')
+    // change to "de" locale
+    await browser.elementByCss("[href='/de']").click()
+    const staticLink = await browser.waitForElementByCss("[href='/de/static']")
+    expect(await browser.elementById('current-locale').text()).toBe(
+      'Current locale: de'
+    )
+
+    // navigate to static route
+    await staticLink.click()
+
+    // the locale should still be "de"
+    expect(await browser.elementById('static-locale').text()).toBe('Locale: de')
+  })
+})

test/e2e/i18n-navigations-middleware/middleware.js

@@ -0,0 +1,6 @@
+import { NextResponse } from 'next/server'
+
+export const config = { matcher: ['/foo'] }
+export async function middleware(req) {
+  return NextResponse.next()
+}

test/e2e/i18n-navigations-middleware/next.config.js

@@ -0,0 +1,9 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+module.exports = {
+  i18n: {
+    defaultLocale: 'default',
+    locales: ['default', 'en', 'de'],
+  },
+}

test/e2e/i18n-navigations-middleware/pages/dynamic/[id].js

@@ -0,0 +1,11 @@
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function Dynamic({ locale }) {
+  return <div id="dynamic-locale">Locale: {locale}</div>
+}

test/e2e/i18n-navigations-middleware/pages/index.js

@@ -0,0 +1,37 @@
+import Link from 'next/link'
+
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function Home({ locale }) {
+  return (
+    <main
+      style={{
+        display: 'flex',
+        flexDirection: 'column',
+        gap: '20px',
+      }}
+    >
+      <p id="current-locale">Current locale: {locale}</p>
+      Locale switch:
+      <Link href="/" locale="default">
+        Default
+      </Link>
+      <Link href="/" locale="en">
+        English
+      </Link>
+      <Link href="/" locale="de">
+        German
+      </Link>
+      <br />
+      Test links:
+      <Link href="/dynamic/1">Dynamic 1</Link>
+      <Link href="/static">Static</Link>
+    </main>
+  )
+}

test/e2e/i18n-navigations-middleware/pages/static.js

@@ -0,0 +1,11 @@
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function Static({ locale }) {
+  return <div id="static-locale">Locale: {locale}</div>
+}