I am using German and it's working fine! Look at the main.inc.php there are two options you have to edit.
// the default locale setting
$rcmail_config['locale_string'] = 'de';
and
// These languages can be selected for spell checking.
// Configure as a PHP style hash array: array('en'=>'English', 'de'=>'Deutsch');
// Leave empty for default set of Google spell check languages
$rcmail_config['spellcheck_languages'] = NULL;
Greetings Jo_shi