docs: add language meta

Author: pdanpdan

packages/playground/pages/+Head.vue

@@ -9,6 +9,7 @@ const logoUrl = normalizeHref('logo.svg');
   <meta name="viewport" content="width=device-width, initial-scale=1" />
   <meta name="theme-color" content="#ffffff" media="(prefers-color-scheme: light)" />
   <meta name="theme-color" content="#1a1a1a" media="(prefers-color-scheme: dark)" />
+  <meta http-equiv="content-language" content="en-us" />
   <meta property="og:type" content="website" />
   <meta property="og:url" content="https://pdanpdan.github.io/virtual-scroll/" />
   <link rel="icon" type="image/svg+xml" :href="logoUrl" />