I'm also interest in this, I've turned off simple cache, which works, but probably isnt the best solution.

thanks dear susan ,yes you are right. I've turned off simple cache too, in socilive.com.:(

How have you created the RTL theme? How are you telling your site whether it should use the RTL of LTR theme?

@Juho in elgg 1.8.19 I'm using a custom version of Aalborg theme, have added a css directory inside the theme called aalborg_theme_rtl and call it from Aalborg start.php using some lines from an elgg 1.7.1 plugin (if users chosen language is English the ltr is called, otherwise the rtl is called

//elgg_extend_view('css/elgg', 'aalborg_theme/css');

if (function_exists('elgg_extend_view') || function_exists('extend_view')){
            // As we see form the elgglib.php file in the elgg_extend_view() function, the views are saved in this array
            // Having our plugin run last means we can reset the array and then just use our own CSS file to extend the view
            if (isset($CONFIG->views->extensions['css'])){
                // Clearing the actual array of views
                $CONFIG->views->extensions['css'] = array();
            }    
    // Adding our CSS file. Add all your CSS to the {ELGG_ROOT}/mod/x/views/default/x/css.php file
            if (function_exists('elgg_extend_view') && $current_lang_id != 'en'){
                //elgg_extend_view('css', 'x/css');
                elgg_extend_view('css', 'aalborg_theme_rtl/css');
            } else if (function_exists('elgg_extend_view')) {
                 elgg_extend_view('css', 'aalborg_theme/css');
            }
}

Hi thanks dear friends
@Juho I change some codes in css (element folder) then used this codes in start.php and elgg.php

start.php(in theme) :

$lan_id = get_current_language();

if ($lan_id == 'fa'){
elgg_extend_view('css/elgg', 'themename/css_RIGHT');

}elseif($Lan_id == 'en'){

elgg_extend_view('css/elgg', 'themename/css');
}

in elgg.php :

$lan_id = get_current_language();
if ($lan_id == 'fa'){

echo elgg_view('css/elements/layout_RIGHT', $vars);
echo elgg_view('css/elements/typography_RIGHT', $vars);
echo elgg_view('css/elements/navigation_RIGHT', $vars);
echo elgg_view('css/elements/modules_RIGHT', $vars);
...

}elseif($lan_id == 'en'){

echo elgg_view('css/elements/layout', $vars);
echo elgg_view('css/elements/typography', $vars);
echo elgg_view('css/elements/forms', $vars);
echo elgg_view('css/elements/navigation', $vars);
...

}
thanks .i used in socilive.com

Currently you're overriding the default CSS so the cache has either RTL of LTR version, but never both.

Instead of overriding the CSS, you could use elgg_register_css() to register the RTL CSS and then call it with elgg_load_css() in case the logged in user is using a RTL language. That way the the default CSS gets loaded on every page and RTL CSS if user's language requires it.

(We should really get better RTL support to core...)