Error if folder doesn't exist when using `fallbackLng`

🐛 Bug Report

• We run multiple domains, some of which are using the same defaultLocale
• Next doesn’t support specifying the same defaultLocale for multiple domains
  • https://github.com/vercel/next.js/discussions/37991
  • https://stackoverflow.com/questions/73410976/next-js-i18n-2-domains-with-the-save-default-locale
• To get past this, we specify unique variants of languages e.g. English (en-DE, en-EN) and rely on fallbackLng to serve the correct language using the configuration below:

next-i18next.config.js

module.exports = {
  i18n: {
    defaultLocale: "en",
    locales: ["en", "en-DE", "en-EN"],
  },
  fallbackLng: {
    default: ["en"],
    "en-DE": ["en"],
  },
  debug: process.env.NODE_ENV === 'development',
  reloadOnPrerender: process.env.NODE_ENV === 'development',
};

next.config.js

const { i18n } = require("./next-i18next.config");

/** @type {import('next').NextConfig} */
const nextConfig = {
  reactStrictMode: true,
  swcMinify: true,
  i18n: {
    ...i18n,
    localeDetection: false,
    domains: [
      {
        domain: "next-i18next.com",
        defaultLocale: "en-EN",
        locales: ["en-EN"],
      },
      {
        domain: "de.next-i18next.com",
        defaultLocale: "en-DE",
        locales: ["en-DE"],
      },
    ],
  },
};

module.exports = nextConfig;

The above works, but only if the folder for that fallback language exists, if the folder doesn’t exist we run into this error:

It seems that we’re not checking whether the locale folder exists before we attempt to read from it:

https://github.com/i18next/next-i18next/blob/master/src/serverSideTranslations.ts#L80-L88

To Reproduce

• Checkout this repo featuring the issue: https://github.com/aaimio/next-i18next-bug
• Set up two domains in your /etc/hosts
  • 127.0.0.1 de.next-i18next.com
  • 127.0.0.1 next-i18next.com
• npm install
• npm run dev
• In your browser, visit de.next-i18next.com you will get the same error

Expected behavior

We should first check whether the folder exists before reading from it.

• If default locale is specified, and its folder doesn’t exist, fallback to value in fallbackLng, use that language’s JSON file
• If default locale is specified, and its folder exists, use that language’s JSON file

Your Environment

• Runtime version: v18.4.0
• next-i18next version: 12.0.0
• OS: Mac

[adrai]

should be better with v12.0.1

[adrai]

uups… should be fixed now