I've got a good solution for this issue.

Please visit platform.dnnsoftware.ir (username: host, password: dnnhost) and try to change language from English to Farsi. You'll see entire site will switch from left to right.

Here I want to share my experience with you. There are two steps to achieve rtl and ltr layout on a same portal as follow:

Step 1: adding class="rtl" to body when selected language is Right To Left

Why: Sometimes attributes like align, float, margin,etc is defined in Javascript files and we need to know page direction to set correct value. Also there are times when we want to override css attributes in right to left languages. This step covers situations like these.

How: with a couple of codes to default.aspx.cs (screenshot below):

Step 2: Skin.rtl.css, container.rtl.css and module.rtl.css

 By default DNN loads skin.css, container.css and module.css on page. It may happen to RTL languages as well if two following conditions come true:

1- Selected language is Right To Left
2- .rtl.css file exists (e.g. skin.rtl.css exists beside skin.css in skin directory)

Please download this file and search for dnnsoftware.ir to see changes.