nextjs - 💡(How to fix) Fix Locale detection isn't working correctly after the update from 15.3 to 15.4/15.5 if a domains array is present in next.config [3 comments, 3 participants]

Code Example

This happens on every environment - local, docker, codesandbox.

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:48:41 PST 2026; root:xnu-12377.81.4~5/RELEASE_ARM64_T6041
  Available memory (MB): 24576
  Available CPU cores: 14
Binaries:
  Node: 22.22.0
  npm: 10.9.4
  Yarn: 4.12.0
  pnpm: N/A
Relevant Packages:
  next: 15.5.12
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.7.2
Next.js Config:
  output: N/A

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/determined-zhukovsky-vzgqvh?workspaceId=ws_XMt216w8w9LTvcfWfEEkxN

To Reproduce

Open codesandbox url
check that the version of next is 15.5.12
run pnpm install
run pnpm dev
you can also open the dev tools in codesandbox to see more information
click on different links in the demo nextjs app to open different locales
the locales listed in the "domains" array will end up taking the defaultLocale of the "domain" and not the defaultLocale of the main domain. The locales not listed in the "domains" array still work correctly
as you can see, the correct value is still present in the req[Symbol.for("NextInternalRequestMeta")]; but it doesn't affect anything.

Current vs. Expected behavior

v15.5 detects the "defaultLocale" of the "domain" in the "domains" array, although being ran on the defaultDomain.

Also SSR adds the domain to the href of the next/links (and because of this also hydration errors)

in v15.3 no issues were present - both the defaultDomain and the domains from the domains array had always the correct defaultLocale. Also the next/links had the correct links without adding the extra "domain".

For even more information pls see "Additional context"

Provide environment information

This happens on every environment - local, docker, codesandbox.

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:48:41 PST 2026; root:xnu-12377.81.4~5/RELEASE_ARM64_T6041
  Available memory (MB): 24576
  Available CPU cores: 14
Binaries:
  Node: 22.22.0
  npm: 10.9.4
  Yarn: 4.12.0
  pnpm: N/A
Relevant Packages:
  next: 15.5.12
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.7.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Pages Router, Internationalization (i18n)

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local)

Additional context

Our "default" domain supports all the listed locales. Our other domains (e.g. "example.fr" has only "fr" and "fr-en", "example.nl" has "nl" and "nl-en"), but the same locales are also supported by the default domain.

in the version 15.3 the locale detection worked perfectly for all these cases. On the "real" domains, listed in the "domains" array it worked and still works correctly, but the problem is with the "default" domain, that also supports the same locales.

v15.5 open the preview page in codesandbox default path "/" detects the correct values

locale from GSSP args: de defaultLocale from GSSP args: de locale from NextInternalRequestMeta: de defaultLocale from NextInternalRequestMeta: de router.locale: de router.defaultLocale: de

If you click on a NextLink (CSR) to "/fr" - that is listed in the "domains" array - the router still has the correct value for "router.defaultLocale: de", but the defaultLocale from the args is now wrong (should be "de")

locale from GSSP args: fr defaultLocale from GSSP args: fr locale from NextInternalRequestMeta: fr defaultLocale from NextInternalRequestMeta: de router.locale: fr router.defaultLocale: de

but if you reload (SSR) the same page "/fr" - router.defaultLocale changes to fr and the defaultLocale from the args is also wrong (both should be "de")

locale from GSSP args: fr defaultLocale from GSSP args: fr locale from NextInternalRequestMeta: fr defaultLocale from NextInternalRequestMeta: de router.locale: fr router.defaultLocale: fr

and a hydration error comes up in the console:

so the next/link is adding the domain to the href now.

The locales, that are not listed in the domains, like "/it" - are being detected correctly. Also the "complex" locales for the listed domains - "fr-en" or "nl-en" are working absolute fine.

extent analysis

Problem Summary

The issue is with Next.js version 15.5.12, where the default locale of the main domain is not being detected correctly when navigating to a locale listed in the "domains" array.

Root Cause Analysis

The root cause is likely due to a change in how Next.js handles locale detection in version 15.5.

Fix Plan

To fix this issue, you can try the following steps:

Step 1: Downgrade Next.js to version 15.3

If you're using version 15.5, try downgrading to version 15.3, which is the version where the locale detection worked correctly.

Step 2: Use `next.config.js` to configure locale detection

You can use the next.config.js file to configure how Next.js detects locales. Try adding the following code to your next.config.js file:

module.exports = {
  // ...
  i18n: {
    locales: ['de', 'fr', 'it', 'nl'],
    defaultLocale: 'de',
  },
};

This will configure Next.js to use the de locale as the default locale.

Step 3: Use `getServerSideProps` to set locale

You can use the getServerSideProps function to set the locale for each page. Try adding the following code to your page component:

import { GetServerSideProps } from 'next';

const MyPage = () => {
  // ...
};

export const getServerSideProps: GetServerSideProps = async (context) => {
  const locale = context.locale || 'de';
  return {
    props: {
      locale,
    },
  };
};

This will set the locale for each page to the default locale de.

Verification

To verify that the fix worked, try the following steps:

Run pnpm dev to start the development

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nextjs - 💡(How to fix) Fix Locale detection isn't working correctly after the update from 15.3 to 15.4/15.5 if a domains array is present in next.config [3 comments, 3 participants]

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Error Message

Root Cause

Code Example

Link to the code that reproduces this issue

To Reproduce

Current vs. Expected behavior

Provide environment information

Which area(s) are affected? (Select all that apply)

Which stage(s) are affected? (Select all that apply)

Additional context

extent analysis

Problem Summary

Root Cause Analysis

Fix Plan

Step 1: Downgrade Next.js to version 15.3

Step 2: Use `next.config.js` to configure locale detection

Step 3: Use `getServerSideProps` to set locale

Verification

Still need to ship something?

TRENDING

nextjs - 💡(How to fix) Fix Locale detection isn't working correctly after the update from 15.3 to 15.4/15.5 if a domains array is present in next.config [3 comments, 3 participants]

Recommended Tools

GitHub issue graph ai analysis

Error Message

Root Cause

Code Example

Link to the code that reproduces this issue

To Reproduce

Current vs. Expected behavior

Provide environment information

Which area(s) are affected? (Select all that apply)

Which stage(s) are affected? (Select all that apply)

Additional context

extent analysis

Problem Summary

Root Cause Analysis

Fix Plan

Step 1: Downgrade Next.js to version 15.3

Step 2: Use next.config.js to configure locale detection

Step 3: Use getServerSideProps to set locale

Verification

Still need to ship something?

RELATED_DISCOVERY

TRENDING

Step 2: Use `next.config.js` to configure locale detection

Step 3: Use `getServerSideProps` to set locale