I have created a page where users get the option of either choosing arabic language forum , or english language forum..
Now i have made two default pages one is default.aspx which is RTL and is english and the other
default_ar.aspx is LTR but my problem is that how can i set my default_ar.aspx to arabic.

i.e can i change the language using a url like the ones being done in vbulletin forums.. or in any other way..

Dear friend,
I have localized an arabic version of the forum, but it can not be done in just an RTL conversion in the default page, you will need to define 2 different themes and to edit manually some pages and controls manipulating text direction. In my advice, separate the two forums on the same database and keep the LTR languages away from RTL ones to be safe. Hence adding a simple control to your forumPage rendering in the head table that captures the current page and redirects it to another language should be very easy.
My regards ..