duolingo.user.js

// ==UserScript==
// @name         Duolingo Russian
// @description  Duolingo Russian
// @namespace    bo33b
// @author       bo33b
// @version      1.1
// @match        https://www.duolingo.com/*
// @grant        none
// ==/UserScript==

(function(window, document) {

  var RU = "яшертйуиопасдфгхжклзцчвбнм-щёэюыь-ЯШЕРТЙУИОПАСДФГХЖКЛЗЦЧВБНМ-ЩЁЭЮЫъ";
  var EN = "qwertyuiopasdfghjklzxcvbnm-23478'-QWERTYUIOPASDFGHJKLZXCVBNM-@#$&*\"";

  HTMLInputElement.prototype.insertAtCaret =  HTMLTextAreaElement.prototype.insertAtCaret = function(text) {
    var caret = this.selectionStart, content = this.value;
    if (caret == content.length) return this.value += text;
    this.value = content.substr(0, caret) + text + content.substr(this.selectionEnd);
    this.selectionEnd = this.selectionStart = caret + text.length;
  }

  document.onkeypress = function(ev) {
    var key = ev.key || String.fromCharCode(ev.charCode)
      , el  = document.querySelector('input, textarea');
    if (el && EN.includes(key) && !(el.disabled||ev.ctrlKey||ev.altKey)) {
      el.focus();
      el.spellcheck = true;
      if (el.placeholder == "Type in Russian") {
        el.lang = 'ru';
        el.insertAtCaret(RU[EN.indexOf(key)]);
        el.dispatchEvent(new UIEvent('blur'));
        ev.preventDefault();
        if (el.value.includes('яа')||el.value.includes('тц'))
          document.querySelector('h1').innerHTML+= '<br>Fool!';
      }
    }
    if (key == '+')
      document.querySelector('h1').innerHTML+=
        `<pre style="position:absolute;left:0;font-size:14px">
.      nominative	= subj.	- Я
.      dative		= of self	- МНЕ
.      genitive   	= of  	- МЕНЯ
.      accusative	= d.o.	- МЕНЯ
.      instrument	= tool	- МНОЙ
.      preposition	= in,at...	- МНЕ</pre>`;
  };

})(window, document);