SpoofIntlLocaleAndTimeZone.user.js

// ==UserScript==
// @name         Spoof Intl Locale and Time Zone
// @namespace    https://greasyfork.org/en/users/85671-jcunews
// @version      1.0.1
// @license      AGPLv3
// @author       jcunews
// @description  .
// @match        *://*/*
// @grant        none
// @run-at       document-start
// ==/UserScript==

(ro => {
  //=== CONFIGURATION BEGIN ===
  //All should be valid values. Otherwise websites may choke.
  let fakeLocale   = "en-GB";
  let fakeTimezone = "Europe/London";
  //=== CONFIGURATION BEGIN ===

  ro = Intl.DateTimeFormat.prototype.resolvedOptions;
  Intl.DateTimeFormat.prototype.resolvedOptions = function() {
    let res = ro.apply(this, arguments);
    res.locale = fakeLocale;
    res.timeZone = fakeTimezone;
    return res;
  };
})();