Sync missing key to other locales after formatjs's extraction have done. these locales are come from next.config.js.

@formatjs+cli+^4.8.4.patch

# generated by patch-package 6.4.10
#
# command:
#   npx patch-package @formatjs/cli
#
# declared package:
#   @formatjs/cli: ^4.8.4
#
diff --git a/node_modules/@formatjs/cli/src/extract.js b/node_modules/@formatjs/cli/src/extract.js
index 5f9095e..73dd043 100755
--- a/node_modules/@formatjs/cli/src/extract.js
+++ b/node_modules/@formatjs/cli/src/extract.js
@@ -166,6 +166,8 @@ function extract(files, extractOpts) {
                         }
                         results[id] = msg;
                     }
+                    const runner = require(process.cwd() + '/scripts/extract.js')
+                    runner(results)
                     return [2 /*return*/, (0, json_stable_stringify_1.default)(formatter.format(results), {
                             space: 2,
                             cmp: formatter.compareMessages || undefined,

babel.config.json

{
  "presets": ["next/babel"],
  "plugins": [
    [
      "formatjs",
      {
        "idInterpolationPattern": "[sha512:contenthash:base64:6]",
        "ast": true
      }
    ]
  ]
}

extract.js

const fs = require("fs-extra");
const nextConfig = require("../next.config.js");
const Opencc = require("opencc");
const converter = new Opencc("s2twp.json");
const debug = require("debug")("extract");

/**
 * Extracts the content of the given file to the given directory.
 * @param {{[key: string]:{ defaultMessage: 'string'}}} results
 */
module.exports = function (results) {
  const {
    i18n: { locales },
  } = nextConfig;
  const defaultLocale = "zh-CN";
  debug("extracting results %o ", results);
  //content/locales/en.json
  locales
    .filter((x) => x !== defaultLocale)
    .forEach((locale) => {
      debug("\r\nhandle locale: %s", locale);
      const localePath = `content/locales/${locale}.json`;
      let localeContent = {};
      const isTW = locale === "zh-TW";
      if (!isTW && fs.existsSync(localePath)) {
        localeContent = fs.readJsonSync(localePath);
        debug("has content %o", localeContent);
      }
      debug("is zh-TW: %s", isTW);
      for (const key in results) {
        if (!localeContent[key]) {
          const text = results[key].defaultMessage;
          localeContent[key] = isTW ? converter.convertSync(text) : text;
        }
      }
      debug("new content %o", localeContent);
      fs.writeJsonSync(localePath, localeContent, {
        spaces: 2,
        replacer: null,
      });
    });
};

next.config.js

/** @type {import('next').NextConfig} */
const nextConfig = {
  reactStrictMode: true,
  i18n: {
    locales: ["en", "zh-CN", "zh-TW"],
    defaultLocale: "en",
  },
};

module.exports = nextConfig;

package.json

{
  "name": "may-nine",
  "version": "0.1.0",
  "private": true,
  "scripts": {
    "dev": "next dev",
    "build": "next build",
    "start": "next start",
    "lint": "next lint",
    "test:e2e": "playwright test",
    "extract:i18n": "DEBUG=* formatjs extract '{pages,components,sections}/**/*.{js,ts,tsx}' --format simple --id-interpolation-pattern '[sha512:contenthash:base64:6]' --out-file content/locales/zh-CN.json",
    "compile:i18n": "formatjs compile-folder --ast --format simple content/locales content/compiled-locales",
    "i18n": "npm run extract:i18n && npm run compile:i18n",
    "postinstall": "patch-package"
  },
  "dependencies": {
    "@mantine/core": "^4.2.4",
    "@mantine/hooks": "^4.2.4",
    "@mantine/next": "^4.2.4",
    "next": "12.1.6",
    "react": "18.1.0",
    "react-dom": "18.1.0",
    "react-intl": "^5.25.1",
    "swr": "^1.3.0"
  },
  "devDependencies": {
    "@formatjs/cli": "^4.8.4",
    "@playwright/test": "^1.21.1",
    "@types/node": "17.0.31",
    "@types/react": "18.0.9",
    "@types/react-dom": "18.0.3",
    "babel-plugin-formatjs": "^10.3.20",
    "eslint": "8.15.0",
    "eslint-config-next": "12.1.6",
    "opencc": "^1.1.3",
    "patch-package": "^6.4.7",
    "typescript": "4.6.4"
  }
}

shell.sh

# create a patch for pnpm
npx @milahu/patch-package-with-pnpm-support @formatjs/cli

# extract and compile i18n
pnpm run i18n

# I use opencc to convert Chinese Simplified to Chinese Traditional (use Taiwan's configuration for better translations.)