Use language tags on literal metadata

[coret]

RDF has the ability to indicate the language of a literal with a tag, eg. @en, but this is optional. When harvesting dataset descriptions we encounter literals in properties like name and description without a language tag, literals specified in Dutch (@nl) and literals specified in English (@en). This make the query to search for dataset descriptions extra complex, because you have to take each possibility into account.

I propose to add a recommendation to the requirements to use language tags with all properties with literals and to provide multilingual descriptions (increase findability internationally). When no language tag is given, the crawler will assume the literals are Dutch and add a @nl tag to the literal. This should be described in the requirements and implemented in the crawler. When implemented all literals in the triplestore will have a language tag, at least @nl or @en. The SHACL based Dataset Register validator should give a warning when no language is specified on a literal.