change-google-search-language.user.js

// ==UserScript==
// @name         検索結果の言語切り替えるやつ
// @description  検索結果ページの言語を切り替えるボタンを表示します
// @namespace    hexium310
// @version      1.1.0
// @author       Hexin
// @match        https://www.google.com/search*
// @resource     MaterialIcons https://fonts.googleapis.com/icon?family=Material+Icons
// @grant        GM_addStyle
// @grant        GM_getResourceText
// @downloadURL  https://gist.github.com/hexium310/4382fac313926d4613fe9e510690baa3/raw/change-google-search-language.user.js
// @updateURL    https://gist.github.com/hexium310/4382fac313926d4613fe9e510690baa3/raw/change-google-search-language.user.js
// ==/UserScript==

(() => {
  'use strict';

  GM_addStyle(GM_getResourceText("MaterialIcons"));

  const url = new URL(location.href);
  const searchParams = url.searchParams;

  if (searchParams.get('hl') === 'en') {
    searchParams.set('hl', 'ja');
    searchParams.set('gl', 'jp');
  } else {
    searchParams.set('hl', 'en');
    searchParams.set('gl', 'us');
  }

  const element = document.querySelector('#searchform > :last-child > :last-child');

  const outer = document.createElement('div');
  outer.style.position = 'relative';
  outer.style.alignSelf = 'center';
  outer.setAttribute('aria-selected', 'false');

  const translation = document.createElement('a');
  translation.href= url.toString();
  translation.style.paddingTop = '0';
  translation.style.paddingBottom = '0';

  const translationImage = document.createElement('span');
  translationImage.classList.add('material-icons');
  translationImage.innerText = 'translate';

  translation.insertAdjacentElement('beforeend', translationImage);
  outer.insertAdjacentElement('beforeend', translation);
  element.insertAdjacentElement('beforebegin', outer);
})();