6.4.9 Property: language (and other locations)
https://www.w3.org/TR/2019/WD-vocab-dcat-2-20190528/

dct:LinguisticSystem Resources defined by the Library of Congress (ISO 639-1, ISO 639-2) SHOULD be used. If a ISO 639-1 (two-letter) code is defined for language, then its corresponding IRI SHOULD be used; if no ISO 639-1 code is defined, then IRI corresponding to the ISO 639-2 (three-letter) code SHOULD be used.

Dublin Core, which is the source for this reference, is unclear (AFAICT) in its relationship to BCP47 and language tags. The use of ISO 639 parts 1 and 2 (without reference to part 3) and the lack of support for the various kinds of subtags in BCP47 (not to mention the stability guarantees found in the BCP registry) are somewhat outdated. It's also notable that the description in DC of "LingusticsSystem" seems to be more like language tags than 639 provides.

Some systems (e.g. library technologies) might be limited to the 639 codes simply because they are intended for a specific industry segment (which is fine, since BCP47 includes the 639 codes). For general interchange, however, it would be better if DCAT permitted language tags and not just 639 codes, as this is the modern standard and used broadly on the Internet/Web.

[This comment is part of the I18N horizontal review.]