diff --git a/pages/_document.tsx b/pages/_document.tsx
new file mode 100644
index 0000000..b2fff8b
--- /dev/null
+++ b/pages/_document.tsx
@@ -0,0 +1,13 @@
+import { Html, Head, Main, NextScript } from "next/document";
+
+export default function Document() {
+  return (
+    <Html lang="en">
+      <Head />
+      <body>
+        <Main />
+        <NextScript />
+      </body>
+    </Html>
+  );
+}
diff --git a/scripts/seo-build.test.mjs b/scripts/seo-build.test.mjs
index 67f95c7..99f4bc5 100644
--- a/scripts/seo-build.test.mjs
+++ b/scripts/seo-build.test.mjs
@@ -18,6 +18,7 @@ function links(html) {
 for (const route of routes) {
   test(`built metadata: ${route}`, () => {
     const html = read(`.next/server/pages${route === "/" ? "/index" : route}.html`);
+    assert.match(html, /<html\b[^>]*\slang="en"(?:\s|>)/, "English pages must declare their document language");
     const tags = links(html);
     const { canonicalUrl, chineseUrl } = getPageUrls(route);
     assert.deepEqual(tags.filter((link) => link.rel === "canonical").map((link) => link.href), [canonicalUrl]);
diff --git a/scripts/seo.test.mjs b/scripts/seo.test.mjs
index d9b52b0..35e5a59 100644
--- a/scripts/seo.test.mjs
+++ b/scripts/seo.test.mjs
@@ -8,6 +8,10 @@ import { getPageUrls, hasChineseTranslation, normalizePath } from "../lib/seo.mj
 const root = new URL("../", import.meta.url);
 const read = (path) => readFileSync(new URL(path, root), "utf8");
 
+test("English document declares its language before client rendering", () => {
+  assert.match(read("pages/_document.tsx"), /<Html\s+lang="en">/);
+});
+
 test("canonical paths exclude queries, fragments, and trailing slashes", () => {
   for (const path of [undefined, "", "/", "/?utm_source=test#top"]) {
     assert.equal(normalizePath(path), "/");