RSS Validation fails

I've a little problem with validating an RSS feed from PunBB (using http://feedvalidator.org)

it returns: Your feed appears to be encoded as "iso-8859-1", but your server is reporting "US-ASCII"

I think this is due to
/Lang/English/common.php

includes

        $locale = 'en_US.US-ASCII';
        break;
    default:
        $locale = 'en_US';
        break;
and
        'lang_encoding'    =>'iso-8859-1',

how do I fix these so they are both iso-8859-1' ?