Coda-RTL.user.js

// ==UserScript==
// @name         Coda-RTL
// @namespace    http://tampermonkey.net/
// @version      0.2
// @description  right to left coda.io documents!
// @author       You
// @match        https://coda.io/d/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    var docName, urlPath, currentHash, domLoadedInterval;

    setTimeout(function(){
        console.log("tampermonkey Actived :-)");
        var urlPath = location.pathname.replace("/d/","");
        var docName = urlPath.substring(0, urlPath.indexOf("_"));
        var currentHash = location.hash;

        if(docName == ""){
            console.log("page is RTL");
            addNewStyles();

            domLoadedInterval = setInterval(function(){
                init();
            }, 1000);

            var hashChangeInterval = setInterval(function(){
                var urlPath = location.pathname;
                var sectionName = urlPath.substring(urlPath.lastIndexOf("/") +1, urlPath.lastIndexOf("_"));
                if(location.hash != currentHash){
                    init();
                    console.log("section changed");
                    currentHash = location.hash;
                }
            },1000);
        }
        else{
            console.log("page is LTR");
        }
    } ,3000);

    function init(){
        var rootElement = document.getElementsByClassName("editable--root--m-Hp5xET")[0];

        if(rootElement)
        {
            rootElement.setAttribute("dir", "rtl");
            console.log("root element direction changed to RTL!");
            clearInterval(domLoadedInterval);
        }
    }

    function addNewStyles(){
        var styleElement = document.createElement("style");
        styleElement.type= "text/css";

        var sidebar = '.Js6D48p4{ left: auto; right: 0; }'
        var content = '.kr-document-root .kr-canvas-section-viewport{margin-left: initial; margin-right: 280px;}'
        var table = '.HVtqADz6{ direction: rtl; }'
        var paragrapg = '.HVtqADz6 > * { text-align: right; }'
        var header = 'header { margin-left: initial !important ; margin-right: 285px !important; } header .jpSfAoNy { flex-direction: row-reverse; }'
        var topIconsContainer = 'header .jpSfAoNy div:nth-child(2) { flex-direction: row-reverse; }'
        var userIcon = 'header .jpSfAoNy div:nth-child(2) div[data-is-transparent-container]:last-child span { padding-left: 0; }'
        var gearIcon = 'header .jpSfAoNy div:nth-child(2) span[data-coda-logging-id="settings"] { margin-left: 10px; }'
        var commentBox = 'div[data-coda-ui-id="commentRail"]{ position: absolute; right: initial; height: 100%; } div[data-coda-ui-id="commentRail"]:hover{ border-left: 0; border-right: 1px solid rgb(224, 224, 224);}'
        var commentArrowIcon = 'div[data-coda-ui-id="commentRail"] .icon{ transform: rotateY(180deg); }'
        var pageOptionContainer = '.I_GqfUwz{ flex-direction: row-reverse; }'

        var styleText = document.createTextNode(sidebar + content + table + paragrapg + header + commentBox + commentArrowIcon + pageOptionContainer + topIconsContainer + userIcon + gearIcon);
        styleElement.appendChild(styleText);
        var head = document.getElementsByTagName("head")[0];
        head.appendChild(styleElement);
    }

})();

You can just install TamperMonkey extension and follow the instruction.

@CyRamos