When trying to change the language to a RTL language the PHP CSS parser runs out of memory. It tries to exceed the standard allocation size of 128mb as defined in the php.ini file which causes the server to return a 500 error.

To replicate:

• Make sure your php.ini file has the memory limit set to 128mb
• Use clean theme
• Install an RTL language pack
• Make sure your cache is cleared so that parser needs to rebuild it
• Change the language to the new RTL language

To work around:
Increase the memory limit in php.ini from 128mb to something larger

The error in my apache logs is:
[Thu Nov 24 02:07:31.101845 2016] [:error] [pid 11843] [client 10.0.2.2:64808] PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 72 bytes) in /home/vagrant/moodles/stable_master/moodle/lib/php-css-parser/Parser.php on line 701

The error in the browser console is:
http://<moodle_url>/stable_master/theme/styles.php/clean/1479953989/all-rtl