metamask-extension/app/scripts/lib/get-first-preferred-lang-code.js

const extension = require('extensionizer')
const promisify = require('pify')
const allLocales = require('../../_locales/index.json')

const getPreferredLocales = extension.i18n ? promisify(
  extension.i18n.getAcceptLanguages,
  { errorFirst: false }
) : async () => []

const existingLocaleCodes = allLocales.map(locale => locale.code.toLowerCase().replace('_', '-'))

/**
 * Returns a preferred language code, based on settings within the user's browser. If we have no translations for the
 * users preferred locales, 'en' is returned.
 *
 * @returns {Promise<string>} Promises a locale code, either one from the user's preferred list that we have a translation for, or 'en'
 *
 */
async function getFirstPreferredLangCode () {
  let userPreferredLocaleCodes = await getPreferredLocales()
  
  // safeguard for Brave Browser until they implement chrome.i18n.getAcceptLanguages
  // https://github.com/MetaMask/metamask-extension/issues/4270
  if(!userPreferredLocaleCodes){
    userPreferredLocaleCodes = []    
  }

  const firstPreferredLangCode = userPreferredLocaleCodes
    .map(code => code.toLowerCase())
    .find(code => existingLocaleCodes.includes(code))
  return firstPreferredLangCode || 'en'
}

module.exports = getFirstPreferredLangCode