By adding an own domain to __(), the use of the WordPress ‘default’ domain is avoided which tries to translate the labels using e.g. the general strings of admin backend, which is bad.
With an own domain it would be possible to create a language file (by hand!) for the label translations. An automated tool won’t work here, see also example 2 here.
Best choice is to remove the __() and to not auto-translate labels.
If translation plugins like WPML are used, an own fieldgroup for each language has to be added anyway, so the labels can be set as wanted by the user.
