A vocabulary tree node based on l10nvocabulary or l10nxvocabulary directory.
These schemas store translations in columns of the form label_xx_XX or
label_xx. The label of a node is retrieved from column label_xx_XX (where
xx_XX is the current locale name) if it exists, from column label_xx (where
xx is the current locale language) else. If this one doesn't exist either,
the english label (from label_en) is used.