fix: do not add locale prefix to api route on NextURL (#36118)

Fixes #35694

## Bug

- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/server/web/next-url.ts

@@ -102,7 +102,8 @@ export class NextURL {
 
     if (
       this[Internal].locale?.locale &&
-      i18n?.defaultLocale !== this[Internal].locale?.locale
+      i18n?.defaultLocale !== this[Internal].locale?.locale &&
+      !this.hasPathPrefix('/api')
     ) {
       pathname = `/${this[Internal].locale?.locale}${pathname}`
     }
@@ -114,6 +115,11 @@ export class NextURL {
     return pathname
   }
 
+  private hasPathPrefix(prefix: string) {
+    const pathname = this[Internal].url.pathname
+    return pathname === prefix || pathname.startsWith(prefix + '/')
+  }
+
   public get locale() {
     return this[Internal].locale?.locale ?? ''
   }

test/unit/web-runtime/next-url.test.ts

@@ -188,3 +188,25 @@ it('allows to clone a new copy', () => {
   expect(url.toString()).toEqual('http://localhost/root/es/bar')
   expect(clone.toString()).toEqual('http://localhost/root-test/es/test')
 })
+
+it('does not add locale for api route', () => {
+  const url = new NextURL('http:///localhost:3000/api', {
+    i18n: {
+      defaultLocale: 'en',
+      locales: ['en', 'es', 'fr'],
+    },
+  })
+  url.locale = 'fr'
+
+  let expected = 'http://localhost:3000/api'
+  expect(url.href).toEqual(expected)
+  expect(url.toString()).toEqual(expected)
+  expect(url.toJSON()).toEqual(expected)
+
+  url.pathname = '/api/hello'
+
+  expected = 'http://localhost:3000/api/hello'
+  expect(url.href).toEqual(expected)
+  expect(url.toString()).toEqual(expected)
+  expect(url.toJSON()).toEqual(expected)
+})