@betablue: Did you checked the code in the plugin, that all language-assignments are correct as berberber wrote ?
For detecting EN-US, you need e.g. an entry like this:
PHP Code:
case 'en-us':
$_REQUEST['langid'] = X;
break;
X must be replacced by the language-id in your forum.
The same you must do for all other languages that you want to have detected and assigned to an installed language.
I will test it with en-us setting in Explorer at the weekend, when I am back at home.
J?rg