I'm not exactly sure what's happening here, nor did I install the addon. But looking at your queries real quick,
PHP Code:
if ($language_abr == $default_language_abr) {
$query = "select tag from ".$pixelpost_db_prefix."tags group by tag order by tag";
} else {
$query = "select alt_tag from ".$pixelpost_db_prefix."tags group by tag order by alt_tag";
}
Shouldn't you group the second query by alt_tag?