mdn-always-en-US.user.js

// ==UserScript==
// @name    MDN always in en-US
// @version 1
// @grant   none
// @match   https://developer.mozilla.org/*
// @run-at  document-start
// ==/UserScript==

(function () {
  const url = document && document.location && document.location.host;
  console.debug({ path });

  if (url !== "developer.mozilla.org") return;

  const engLangTag = "en-US";
  const path = document.location.pathname;
  console.debug({ path });

  if (path.substring(0, `/${engLangTag}`.length) === `/${engLangTag}`) return;

  console.info("Changing MDN language to English");

  const newPath = "/"
    .concat(engLangTag)
    .concat(path.substring(path.split("/").filter(Boolean)[0].length + 1));
  console.debug({ newPath });

  window.location.replace(newPath);
})();