How to use:

Step 1: Install a script manager if you don't have one already. I use TamperMonkey, the most popular script manager on Firefox. I will be assuming you're using that script manager, too. You may want to give it permission to run in private windows.

Step 2: Click on TamperMonkey in your list of browser extensions, make sure it's enabled, then click "create a new script".

Step 3: Copy-paste the following script.

// ==UserScript==
// @name         Wikipedia Language Redirector
// @version      v3
// @match        https://en.wikipedia.org/*
// @description  Redirects wikipedia pages to alternate language pages in order of predefined preference
// @author       Enjoyer_of_Games
// @author       Edie
// @run-at       document-idle
// @license      AGPL 3+
// ==/UserScript==

const langBtn = document.querySelector('#p-lang-btn');

if (langBtn) {
    let LanguageList = ['nb', 'nn', 'da', 'sv', 'eo', 'tok', 'ru', 'ja', 'zh'];
    for (const langCode of LanguageList) {
        let linktarget = document.querySelector(`a.interlanguage-link-target[lang="${langCode}"]`);
        if(linktarget){
            let langLink = linktarget.href;
            if (langLink){
              console.log(window.location);
              console.log(`Navigating to language: ${langCode}`);
              window.location.href = langLink;
              break;
            }
            else {
               console.warn(`Could not find language link for "${langCode}"`);
            }
        }
    }
}
else {
    console.log('Could not find the #p-lang-btn element');
}

Step 4: Modify the script at the following points:

Point 1:

// @match        https://en.wikipedia.org/*

Replace the "en" in https://en.wikipedia.org/* with the Wikipedia language code of whatever your first language is.

Point 2:

    let LanguageList = ['nb', 'nn', 'da', 'sv', 'eo', 'tok', 'ru', 'ja', 'zh'];

Replace 'nb', 'nn', 'da', 'sv', 'eo', 'tok', 'ru', 'ja', 'zh' with the Wikipedia language codes of your target languages in order of priority, using the same syntax. Note that you need to use "nb" as the language code for Norwegian Bokmål, not "no". Edie says to use ISO 639 codes instead of Wikipedia language codes, because of the nb/no issue, but I'm not sure if this is accurate.

Step 5: Click "file" → "save", then under TamperMonkey's "installed userscripts" tab make sure that the Wikipedia Language Redirector script is enabled.

Step 6: Test it out and see if it works!