In Drupal 6 the default language is an object stored in a variable. So if you want to have different default languages for each domain/subdomain, you need:

1. Different settings.php files, one for each domain, see the handbook about multisite set ups, http://drupal.org/getting-started/6/install/multi-site

2. To properly set up the language default variable in settings.php. For English as the default language it would be: