By default the language of the spell checker in English. You can change it with the following snippet (which allows English and German)

 
function fb_mce_external_languages($initArray){ 
$initArray['spellchecker_languages'] = '+German=de, English=en'; 
 
return $initArray; 
} 
add_filter('tiny_mce_before_init', 'fb_mce_external_languages');