How will dct:language be used in practice? In the reference document, it is said to be expected with instances of dct:LinguisticSystem. Are the ISO 639 standard codes falling in scope of this class? In practice, I expect that dct:language will be used with simple string codes from ISO, like "en". But using a literal in places where a string is expected could create issues in the RDF/OWL technological stack. It could just be simpler if no class was formally expected for dct:language.
This issues has been reported by Antoine Isaac: