Ahhh, thanks.

I dont know, but is it neccessary to set "normal" inputs/textareas ín RTL-Mode, too?

I've created the following file:

cms/css/backend-rtl.css

input, textarea, select {
    direction: rtl !important;
}

And added to your condition:

// Switch directionality for arabic sites in Backend Editors
if(stristr($_SERVER['REQUEST_URI'], 'ar_SA') !== FALSE || stristr($_SERVER['REQUEST_URI'], 'ar-')){
   HtmlEditorConfig::get('cms')->setOption('directionality', 'rtl');
   LeftAndMain::require_css('cms/css/backend-rtl.css');
}

And now the form fields are in RTL mode, too.

Is this correct for "arabic mode" ?

Rob