proslji.js

// ==UserScript==
// @name     Proslji
// @version  1
// @grant    none
// @include  https://*.slack.com/*
// ==/UserScript==

var fixit = {"proslji" : "prošli", 
             "prosljeg":"prošlog", 
             "prosljem":"prošlom", 
             "prosljim":"prošlim", 
             "proslje":"prošle", 
             "proslju":"prošlu"};

var keys = Object.keys(fixit);
for(var i = 0; i < keys.length; i++){
  var replacement = fixit[keys[i]];
  fixit[keys[i]] = [new RegExp(keys[i], "ig"), replacement];
}

var chatBox = document.getElementById("col_messages");
var re = new RegExp("\\b(" + keys.join("|") + ")\\b", "ig");

var callback = function(mutationsList, observer) {
  for(var mutation of mutationsList) {
    for(var node of mutation.addedNodes){
      
      var messages = node.querySelectorAll("span.c-message__body");			
      for(var message of messages){
        var m = message.textContent.match(re);
        if(m){
          
          for(var i = 0; i < m.length; i++){
            var pair = fixit[m[i].toLowerCase()]
            
            var r = pair[0];
            message.textContent = message.textContent.replace(r, pair[1]);
          }
        }  
      }          
    }
  }
}

var config = { attributes: false, childList: true, subtree: true };

var observer = new MutationObserver(callback);
observer.observe(chatBox, config);