Is there a open source .NET CMS that has support for Arabic language? Anyone here has an experience with using one?
I believe umbraco supports Arabic.
I recently did some research into how much work it would be to convert one of our existing multi lingual sites to include Arabic.
The biggest issue was making the back end WYSIWYG editor (TinyMCE) RTL. However this turned out to be quite simple - just a case of enabling the RTL buttons in the config. Found here http://forum.umbraco.org/yaf_postst9084_How-can-we-use-incomplete-xml-langauge-file-in-umbraco.aspx
I'm not sure whether umbraco's back end actually can be displayed in Arabic.
Most .NET based stuff are using Unicode so they (in theory) should support any languages. Most of them are easily translatable, too.
So, you're good to go with any of them. Perhaps you should look at DotNetNuke or Umbraco or Sense/Net, those are big ones.