DeutscheLetters.user.js

// ==UserScript==
// @name         Deutsche letters
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Change letters to umlaut letters
// @author       Nikolay Dozmorov
// @match        http://*/*
// @match        https://*/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    var mapping = {
        "a": "ä",
        "o": "ö",
        "u": "ü",
        "s": "ß",
        "A": "Ä",
        "O": "Ö",
        "U": "Ü"
    };

    document.body.addEventListener('keyup', function(e) {
        if ((e.target.tagName == "INPUT" || e.target.tagName == "DIV") && e.altKey) {
            var newKey = mapping[e.key];

            if (newKey) {
                var off = 0;
                if (e.target.tagName == "INPUT") {
                    off = e.target.selectionStart;
                    e.target.value = e.target.value.slice(0, off) + newKey + e.target.value.slice(off);
                    e.target.setSelectionRange(off + 1, off + 1);
                }
                if (e.target.tagName == "DIV") {
                    var sel = window.getSelection();
                    var range = sel.getRangeAt(0);
                    off = range.startOffset;

                    e.target.innerHTML = e.target.innerHTML.slice(0, off) + newKey + e.target.innerHTML.slice(off);
                    range.setStart(e.target.childNodes[0], off + 1);
                    range.collapse(true);
                    sel.removeAllRanges();
                    sel.addRange(range);
                }
                e.preventDefault();
            }
        }
    });
})();