Allow one prefLabel per language per concept

[leskneebone]

VocPub specification limits prefLabel to exactly one per concept. This is a variation from skos where one prefLabel per language per concept is allowed.

The variation may be an attempt to keep skos even more simple. But this variation makes things difficult for multilingual vocabulary projects, especially where the second (or third, fourth etc) languages are not a 'nice to have', but rather required as part of a full set that may be applied in some information context where that language is used as default in other reference and catalogue data.

An example context where multiple prefLabel instances are needed is the Indigenous Data Network. Some IDN vocabularies include @aus language tags, as well as @en tags. Many labels don't have a tag (this needs fixing!). The skos:prefLabel "Aboriginal English" currently has (incorrectly) an @aus tag that should be @en. Should @aus tags be added as altLables, we wouldn't be able to determine which is the better (or indeed, 'preferred') label.

An example where multiple languages are used for both prefLabel and altLabel is Biosecurity Thesaurus:

prefLabel: "Biological invasions"@en
prefLabel: "Invasi hayati"@id
altLabel: "Bioinvasion"@en
altLabel: "Bioinvasi"@id

This issue is the subject of discussion: Kurrawong/vocedit#15 (comment)