In the config file of FH, if the language is 'fr-utf8' (all languages with '-utf8') the class.Editor.php at line 38 indicate a bad language not exist in FCKeditor.
line 38 :
<?php
$this->_oEditor->Config['AutoDetectLanguage'] = false ;
$this->_oEditor->Config['DefaultLanguage'] = $oForm->_lang;// fr-utf8 not exist in fck editor language
?>