Hi masson!
I solved the problem by adding and changing the folowing code in the sapphire/core/model/fieldtypes/Date.php file:

function Format($formattingString) {

if($this->value) return date($formattingString, strtotime($this->value));

function FormatDate($format) {

if ($this->value) $mydate = strftime($format, strtotime($this->value));

return utf8_encode($mydate);

Call the function width $Date.FormatDate(%B %Y).

That should work out!
Caution: To see other languages the setlocal for the language must be installed on your server!

the _config.php needs this to use it (for German e.g.):
i18n::set_locale('de_DE');
setlocale (LC_ALL, 'de_DE@euro', 'de_DE', 'de', 'ge');