Next.js Multilingual Blog SEO — Why Google Ignores /ko, and Two Fixes

Google Search Console flagged this.

Duplicate, Google chose different canonical than user
Affected pages: https://backtodev.com/ko, https://backtodev.com/ko/contact

I had clearly set canonical to https://backtodev.com/ko, but Google ignored it and picked the /en version as the representative URL instead.

There were two causes.

Cause 1 — missing x-default hreflang

Setting up hreflang on a multilingual site usually looks like this.

alternates: {
  canonical: `${BASE_URL}/${locale}`,
  languages: {
    ko: `${BASE_URL}/ko`,
    en: `${BASE_URL}/en`,
  },
},

Notice ko and en are there, but x-default is missing.

x-default is the signal telling Google "here's the default URL to use when no language matches." Without it, Google has to decide for itself which of /ko and /en is the "true representative" — and that decision might not match your intent.

In fact, this blog's default language is Korean, but with no x-default, Google chose /en as the canonical, pushing /ko down as a "duplicate page."

Cause 2 — insufficient handling of isFallback posts

This blog has /en/posts/[slug] URLs even for posts that only exist in Korean. With no English translation, it just shows the Korean content as-is (isFallback).

From Google's perspective, then:

https://backtodev.com/ko/posts/some-post → Korean content
https://backtodev.com/en/posts/some-post → the exact same Korean content

Two URLs, identical content → flagged as a duplicate page.

Locking canonical to /ko doesn't stop Google from crawling and attempting to index the /en URL itself. A stronger signal is needed.

Fix 1 — add x-default

Added x-default to every page's generateMetadata.

Homepage (app/[locale]/page.tsx):

alternates: {
  canonical: `${BASE_URL}/${locale}`,
  languages: {
    ko: `${BASE_URL}/ko`,
    en: `${BASE_URL}/en`,
    "x-default": `${BASE_URL}/ko`,  // added
  },
},

Same for the posts list, about, and contact pages:

languages: {
  ko: `${BASE_URL}/ko/posts`,
  en: `${BASE_URL}/en/posts`,
  "x-default": `${BASE_URL}/ko/posts`,  // added
},

Once rendered, this produces:

<link rel="alternate" hreflang="ko" href="https://backtodev.com/ko" />
<link rel="alternate" hreflang="en" href="https://backtodev.com/en" />
<link rel="alternate" hreflang="x-default" href="https://backtodev.com/ko" />

Now Google clearly understands "the default language is Korean."

Fix 2 — add noindex to isFallback posts

Canonical alone isn't enough. Added noindex so Google never indexes the Korean-content /en posts at all.

// app/[locale]/posts/[slug]/page.tsx
return {
  title: post.title,
  description: post.description,
  ...(post.isFallback && { robots: { index: false, follow: false } }),  // added
  alternates: {
    canonical: canonicalUrl,
    languages: {
      ko: `${BASE_URL}/ko/posts/${slug}`,
      ...(otherPost && !otherPost.isFallback
        ? { en: `${BASE_URL}/en/posts/${slug}`, "x-default": `${BASE_URL}/ko/posts/${slug}` }
        : { "x-default": `${BASE_URL}/ko/posts/${slug}` }),
    },
  },
  // ...
};

The tag rendered when isFallback is true:

<meta name="robots" content="noindex, nofollow" />

Google sees this tag and excludes that URL from indexing entirely. A stronger signal than canonical.

noindex vs. canonical → /ko — which is better?

Approach	Pros	Cons
`canonical → /ko`	simple to implement	Google can ignore it. Also creates a URL contradiction (English URL → Korean canonical)
`noindex`	reliably blocks indexing	that URL will never appear in search results at all

For a page with no English translation, noindex is the right call. It's a clear signal: "this page isn't ready yet." Once English content is added later, just remove the noindex.

Scope of application, summarized

Page	x-default	noindex
Home (`/[locale]`)	✅ added	❌ not needed (has English content)
Post list	✅ added	❌ not needed
about	✅ added	❌ not needed
contact	✅ added	❌ not needed
Post detail (isFallback)	✅ added	✅ added
Post detail (normal)	✅ added	❌ not needed

Summary

Google Search Console: /ko duplicate page error
    ↓
Cause 1: missing x-default hreflang
        → Google arbitrarily chose canonical between /ko and /en
Cause 2: isFallback posts had no noindex
        → Korean content on an /en URL → flagged as duplicate
    ↓
Fix:
  All pages → add "x-default": /ko
  isFallback posts → robots: { index: false, follow: false }
    ↓
Request re-inspection in Google Search Console → resolves within days

When building a multilingual Next.js blog, x-default is an easy thing to forget in your hreflang setup. It's tempting to think just ko and en are enough, but without x-default, Google has no way of knowing which one is actually the default.