Ensure API routes are not available under the locale (vercel#26629)

This ensures API routes are not available under the locale path since API routes don't need to be localized and we don't provide the locale to the API in any way currently so the user wouldn't be aware if the localized API route was visited instead of the non-localized. 

Fixes: vercel#25790

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added

Author: ijjk

packages/next/next-server/server/next-server.ts

@@ -465,6 +465,13 @@ export default class Server {
           pathname: localePathResult.pathname,
         })
         ;(req as any).__nextStrippedLocale = true
+
+        if (
+          localePathResult.pathname === '/api' ||
+          localePathResult.pathname.startsWith('/api/')
+        ) {
+          return this.render404(req, res, parsedUrl)
+        }
       }
 
       // If a detected locale is a domain specific locale and we aren't already

test/integration/i18n-support/test/shared.js

@@ -37,7 +37,7 @@ async function addDefaultLocaleCookie(browser) {
 }
 
 export function runTests(ctx) {
-  it.only('should have domainLocales available on useRouter', async () => {
+  it('should have domainLocales available on useRouter', async () => {
     const browser = await webdriver(ctx.appPort, `${ctx.basePath || '/'}`)
     expect(
       JSON.parse(await browser.elementByCss('#router-domain-locales').text())
@@ -1146,44 +1146,54 @@ export function runTests(ctx) {
     for (const locale of locales) {
       const res = await fetchViaHTTP(
         ctx.appPort,
-        `${ctx.basePath || ''}${
-          locale === 'en-US' ? '' : `/${locale}`
-        }/api/hello`,
+        `${ctx.basePath || ''}/${locale}/api/hello`,
         undefined,
         {
           redirect: 'manual',
         }
       )
 
-      const data = await res.json()
-      expect(data).toEqual({
-        hello: true,
-        query: {},
-      })
+      expect(res.status).toBe(404)
     }
+
+    const res = await fetchViaHTTP(
+      ctx.appPort,
+      `${ctx.basePath || ''}/api/hello`
+    )
+
+    const data = await res.json()
+    expect(data).toEqual({
+      hello: true,
+      query: {},
+    })
   })
 
   it('should visit dynamic API route directly correctly', async () => {
     for (const locale of locales) {
       const res = await fetchViaHTTP(
         ctx.appPort,
-        `${ctx.basePath || ''}${
-          locale === 'en-US' ? '' : `/${locale}`
-        }/api/post/first`,
+        `${ctx.basePath || ''}/${locale}/api/post/first`,
         undefined,
         {
           redirect: 'manual',
         }
       )
 
-      const data = await res.json()
-      expect(data).toEqual({
-        post: true,
-        query: {
-          slug: 'first',
-        },
-      })
+      expect(res.status).toBe(404)
     }
+
+    const res = await fetchViaHTTP(
+      ctx.appPort,
+      `${ctx.basePath || ''}/api/post/first`
+    )
+
+    const data = await res.json()
+    expect(data).toEqual({
+      post: true,
+      query: {
+        slug: 'first',
+      },
+    })
   })
 
   it('should rewrite to API route correctly', async () => {