passed/static/js/lang.js

const LANGUAGE_KEY = "language";
const languages = {};
let language = "en";

/**
 * @param {string} lang
 */
async function setLanguage(lang) {
  if (languages[lang] == undefined) {
    const res = await fetch(`/lang/${lang}.json`);
    const json = await res.json();

    languages[lang] = json;
  }

  language = lang;
  translatePage();
  localStorage.setItem(LANGUAGE_KEY, lang);
}

/** Translates a key
 * @param {string} key
 * @param {any} args
 * @returns {string}
 */
function translate(key, ...args) {
  const bundle = languages[language];
  let value = bundle[key];
  if (value == undefined) {
    value = key;
  }

  for (let i = 0; i < args.length; i++) {
    value = value.replaceAll(`{${i}}`, args[i]);
  }

  return value;
}

function translatePage() {
  const elements = document.getElementsByTagName("*");
  for (const element of elements) {
    const key = element.getAttribute("t");
    if (key == null) {
      continue;
    }

    element.innerHTML = translate(key);
  }
}

function initLanguageButtons() {
  const buttons = document.querySelectorAll("li.select-language");
  for (const button of buttons) {
    button.addEventListener("click", async (ev) => {
      const self = ev.target;
      const lang = self.getAttribute("data-lang");
      await setLanguage(lang);
    });
  }
}

window.addEventListener("load", async () => {
  let lang = localStorage.getItem(LANGUAGE_KEY);
  if (lang == null) {
    lang = "en";
  }

  await setLanguage(lang);
  initLanguageButtons();
});