wp-config.php:
/**  
 * WordPress Localized Language, defaults to English.  
 *  
 * Change this to localize WordPress. A corresponding MO file for the chosen  
 * language must be installed to wp-content/languages. For example, install  
 * de_DE.mo to wp-content/languages and set WPLANG to 'de_DE' to enable German  
 * language support.  
 */  
define('WPLANG', 'de_DE');  
This part does not seem to work with the “–wpsubdir” option. It works fine with a single site.
Also, I was wondering when and how this feature is incorporated:
      
    