When you want to use a "Multiple directory suggestion" widget for the dc:subjects field (or any other field for that matter), you can't use any of the pre-defined vocabulary that looks potentially usable and are labelled "subject", "subtopic" and "topic". In the best case, in the browser, you have only the first level of the vocabulary.
To display the list as expected you must:
- Set "Activate dbl10n" to yes (though, this one is not related, it's just to get the correct labels)
- But not pickup a vocabulary using the "vocabulary" button
- Instead, add a property, name "directoryName", value "l10nsubjects"
=> It could be better to find a way to let users of Studio use this existing vocabuary in a select2 widget, without this need of manually enter a specify property