The most current released version of UBB.threads is 7.5.9 (January 2015)
/languages/english/generic,php line number two is:
$ubbt_lang['CHARSET'] = "UTF-8";
I cannot find "iso-8859-1" in any other file within the most current version (7.5.9) of UBB.threads, other than the included library, phpmailer 2.0.x