Set default language for domain
Though I'm surprised if there's not an option in the user interface, according to language_default, it appears you could set the default language as a $conf variable in the shared settings.php file.

Something like

settings.php:

switch($_SERVER['SERVER_NAME']){

  case 'www.example.com':

    $conf['language_default'] = (object) array(
      'language' =&gt; 'en', 'name' =&gt; 'English', 'native' =&gt; 'English',
      'direction' =&gt; 0, 'enabled' =&gt; 1, 'plurals' =&gt; 0, 'formula' =&gt; '',
      'domain' =&gt; 'www.example.com', 'prefix' =&gt; '', 'weight' =&gt; 0, 'javascript' =&gt; ''
    );

    // Example per-domain cache prefix if using memcache
    $conf['memcache_key_prefix'] = 'en_';

  break;
  case 'www.example.de':

    $conf['language_default'] = (object) array(
      'language' =&gt; 'de', 'name' =&gt; 'German', 'native' =&gt; 'German',
      'direction' =&gt; 0, 'enabled' =&gt; 1, 'plurals' =&gt; 0, 'formula' =&gt; '',
      'domain' =&gt; 'www.example.de', 'prefix' =&gt; '', 'weight' =&gt; 0, 'javascript' =&gt; ''
    );

    // Ensures cache varies by domain
    $conf['memcache_key_prefix'] = 'de_';

  break;

}


Update: Added example memcache cache key prefix setting - to ensure cache varies per domain, if required.