The cause of this problem is utf8_encode() function in lib/html2text.php.(since 12th/Jun/2009)

function _convert()
{
// Variables used for building the link list
$this->_link_count = 0;
$this->_link_list = '';

$text = trim(stripslashes($this->html));

// Convert <PRE>
$this->_convert_pre($text);

// Run our defined search-and-replace
$text = preg_replace($this->search, $this->replace, $text);
$text = preg_replace_callback($this->callback_search, array(&$this, '_preg_callback'), $text);

// Replace known html entities
// $text = utf8_encode(html_entity_decode($text)); // Here!!!
$text = html_entity_decode($text, ENT_COMPAT, 'UTF-8');

// Remove unknown/unhandled entities (this cannot be done in search-and-replace block)
$text = preg_replace('/&[^&;]+;/i', '', $text);

utf8_encode() function converts characters from ISO-8859-1 to UTF-8.
Then, Multibyte UTF-8 characters were convereted to wrong unexpected characters...