I think it is a design decision and shouldn't be done by me, but the RoundCube designers.
Anyway I searched about implementing support of RTL languages in web applications, especially PHP. Here are a few useful links:

RTL support in WordPress 2.1: http://boren.nu/archives/2007/01/07/rtl-support-in-wordpress-21/
RTL in HTML, XHTML, CSS2, CSS3: http://www.i18nguy.com/markup/right-to-left.html
Language information and text direction (part of HTML 4.01 specification): http://www.w3.org/TR/html401/struct/dirlang.html

I think this is the simplest idea:
Set a global variable in a locale-dependent file which shows this locale is RTL. Then use it wherever a 'dir="RTL"' attribute is needed.
Anyway it is up to designers to decide upon this.


On 10/8/07, Thomas Bruederli < roundcube@gmail.com> wrote:
Mohammad Ebrahim Mohammadi Panah wrote:
> Hello,
> I got the latest code from Subversion and tested it to see how it looks
> in Persian. RoundCube seems to have no support for right-to-left
> languages such as Persian and Arabic. It didn't change the layout to
> right-to-left when I chose Persian or Arabic. Please correct me if I'm
> wrong.
> Is there any plan to implement it?
>
Somebody once announced to implement this but I've never heard again from
that person. This has to be done by somebody who is familiar with
right-to-left websites and the Persian or Arabic language. All of your
developers are either American or European and have no idea about RTL websites.

You are very welcome to contribute the changes that are necessary for this.

Regards,
~Thomas
 

--
         Mohammad Ebrahim Mohammadi Panah