lib/smarty/plugins/function.html_select_date.php : 308
- $_text = isset($month_names) ?
smarty_function_escape_special_chars($month_names[$i]) : ($month_format == "%m"
? $_val : strftime($month_format, $_month_timestamps[$i]));

strftime(..) return an latin1 date in french ... but ut8 is needed !

solution :
add a new line : $_text = utf8_encode($_text);