get_locales.js

// https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

function get_locales() {
  let all = document.getElementsByTagName('pre')[0].innerText
  let regex = /Subtag: (.*)/
  let match
  let matches = []
  while(match = regex.exec(all)) {
    matches.push(match[1])
  }
  return matches
}

let locales = get_locales()
console.log(locales)