JSFiddle - React, Tailwind, and code Playground

HTML

<h1 lang="en">One, two, three</h1>
<h1 lang="de">Eins, zwei, drei</h1>

<p>
    <button id="switch-to-de">Deutsch</button>
    <button id="switch-to-en">Englisch</button>
</p>

CSS

.lang-en :lang(de) { display: none; }
.lang-de :lang(en) { display: none; }

JavaScript

var switchLanguage = function(lang) {
    document.body.className = "lang-" + lang;
};
document.getElementById('switch-to-de').onclick = function() {
    switchLanguage('de');
};
document.getElementById('switch-to-en').onclick = function() {
    switchLanguage('en');
};