[Admin] Add possibility to add custom admin languages


Currently it's not possible to add your own admin language.
By discovering also the config `framework.translator.default_path` https://github.com/pimcore/pimcore/blob/2cf5cf3a00d8996c60e5a149bc78a12a2a38c558/lib/Tool/Admin.php#L51 here, we should be able to provide this functionality quite easily. 