Userscript: Redirect Website to its English Version

redirect-to-english.js

// This userscript redirects you to the English version of a website if it's denoted in the source code.
// Insert any URLs of websites below (after @match), for example https://developer.mozilla.org/* or https://www.php.net/*
// Use multiple @match clauses to enable the script on several domains.

// ==UserScript==
// @name         Redirect to English
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Redirect websites to their English version
// @author       Florian Reuschel <florian@loilo.de>
// @match        [INSERT URL HERE]
// @grant        none
// ==/UserScript==

if (document.documentElement.lang !== 'en') {
    const referrerHost = document.referrer
        ? new URL(document.referrer).hostname
        : undefined
    const currentHost = new URL(location.href).hostname

    if (
        // Don't redirect when directly calling the page
        referrerHost !== undefined &&

        // Don't redirect if coming from another page on the same domain
        referrerHost !== currentHost &&

        // Don't redirect if history state indicates that this page has previously been
        // redirected away from (to maintain ability to go back to the original language)
        !(history.state && history.state.redirectToEnglish)
    ) {
        const englishLink = document.head.querySelector('link[rel="alternate"][hreflang="en"]')

        if (englishLink) {
            history.replaceState({ redirectToEnglish: true }, null)
            location.assign(englishLink.href)
        }
    }
}