feat：Jump to the related 404 page file for the local server when running (#4719)

Now in our current server.js, when the route is invalid, only "Not Found" is returned. Actually to mock what we see from the official site, it seems we should return you a 404 page instead.

Author: SEWeiTung

server.js

@@ -110,6 +110,7 @@ staticFiles.on('add', (filePath) => {
 });
 
 const mainLocale = (selectedLocales && selectedLocales[0]) || 'en';
+let all404NotFoundPageContent = null;
 
 // Initializes the server and mounts it in the generated build directory.
 http
@@ -120,7 +121,43 @@ http
     if (req.url === '/') {
       req.url = `/${mainLocale}`;
     }
-    mount(req, res);
+
+    mount(req, res, async () => {
+      // Here we should handle the 404 page when the route is not found.
+      // 1. Check whether the language is supported or not.
+      // 2. If not, redirect to the default language.
+      // 3. If yes, Save the content and return directly for the next time.
+
+      if (all404NotFoundPageContent === null) {
+        const allSupportedLangs = await fs.readdir(
+          path.join(__dirname, 'locale')
+        );
+        all404NotFoundPageContent = new Map();
+        for (const lng of allSupportedLangs) {
+          all404NotFoundPageContent.set(lng, '');
+        }
+      }
+
+      let currentLng = req.url.split('/')[1];
+
+      if (!all404NotFoundPageContent.has(currentLng)) {
+        currentLng = mainLocale;
+      }
+
+      if (all404NotFoundPageContent.get(currentLng) === '') {
+        const notFoundPagePath = path.join(
+          __dirname,
+          'build',
+          `${currentLng}/404.html`
+        );
+        all404NotFoundPageContent.set(
+          currentLng,
+          await fs.readFile(notFoundPagePath, 'utf8')
+        );
+      }
+
+      res.end(all404NotFoundPageContent.get(currentLng));
+    });
   })
   .listen(port, () => {
     console.log(