Undefined function is_rtl()

WordPress 3.2.1 is running fine.

I installed the atg-styles-dropdown plugin and regardless of whether the plugin is activated or not, when I login to the dashboard, I get the message:

Fatal error: Call to undefined function is_rtl() in /usr/www/users/cjsmall/changemarketer/wp-includes/theme.php on line 1817

The first line in the atg-styles-dropdown.php file is making a call to the add_editor_style() function which resides in the theme.php file, and on line 1817, there is indeed a call to is_rtl().

The is_rtl() function appears to be defined near the end of the wp-includes/locale.php file, although there appear to be two definitions; one inside the WP_Locale class and another outside the class.

I’m guessing that there is a bug in the WordPress code where the theme.php file does not include the locale.php file. So I added the following line to the start of function add_editor_style():

require_once( ABSPATH . ‘/wp-includes/locale.php’ );

This changes the error message to:

Fatal error: Call to a member function is_rtl() on a non-object in /usr/www/users/cjsmall/changemarketer/wp-includes/locale.php on line 348

It should be possible to successfully make a call to add_editor_style() without a fatal error! Does anyone have a clue what is wrong here? Thanks for any insights you can offer.