Ik ben geen expert, maar...

userscript.js

// ==UserScript==
// @name         Ik ben geen expert, maar...
// @namespace    https://floris.debijl.xyz/
// @version      1.0
// @author       @Fdebijl
// @license      GNU Affero General Public License 3.0 https://www.gnu.org/licenses/agpl-3.0.nl.html
// @match        https://twitter.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    const tweetTextSelector =  `div.css-1dbjc4n.r-18u37iz.r-thb0q2 > div.css-1dbjc4n.r-1iusvr4.r-46vdb2.r-1777fci.r-5f2r5o.r-bcqeeo.r-1mi0q7o > div.css-901oao.r-jwli3a.r-1qd0xha.r-a023e6.r-16dba41.r-ad9z0x.r-bcqeeo.r-bnwqim.r-qvutc0 > span`;

    const experts = {
        en: `I'm not an expert, but `,
        nl: `Ik ben geen expert, maar `
    }

    const decapitalize = (string) => {
        return string.charAt(0).toLowerCase() + string.slice(1);
    }


    setInterval(() => {
        [...document.querySelectorAll(tweetTextSelector)].forEach(tweet => {
            if (!tweet.innerText) return;
            if (tweet.attributes.prefixed) return;
            const prefix = experts[tweet.parentElement.attributes.lang.value] || experts.en;
            tweet.innerText = prefix + decapitalize(tweet.innerText);
            tweet.setAttribute('prefixed', true);
        });
    }, 5000);
})();