force-browser-language.user.js

// ==UserScript==
// @name Force Browser Language
// @version 0.7
// @downloadURL https://userscripts.codonaft.com/force-browser-language.user.js
// @match https://*.google.com/*
// ==/UserScript==

(_ => {
'use strict';

const first = lang => lang.split('-')[0]
const lang = navigator.language || 'en';
const url = new URL(window.location.href);
if (!url.searchParams.has('hl') || first(url.searchParams.get('hl')) !== first(lang)) {
  window.stop();
  url.searchParams.set('hl', lang);
  window.location.replace(url);
}
})();