Shouldn't CRM_Core_PseudoConstant::get localize parameter be setted to TRUE by default?
Case statuses are user-defined option values (configurations). If you have installed CiviCRM initially in English, then switched the language, you will have to edit/translate the case statuses manually in: Administer > Case > Case statuses (/civicrm/admin/options/case_status?reset=1).