Appendix 4. ISO 639-1, ISO 639-2 and ISO 639-3 implementation examples

ISO 639-1 and ISO 639-2

Language property in the Data Catalog Vocabulary (DCAT) – Version 2 (W3C Recommendation 04 February 2020): 

Range: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.
Usage note:Repeat this property if the resource is available in multiple languages.

The same wording is included in the Data Catalog Vocabulary (DCAT) – Version 3

W3C Working Draft 10 May 2022.

Codes for the Representation of Names of Languages arranged alphabetically by alpha-3/ISO 639-2 Code: http://www.loc.gov/standards/iso639-2/php/code_list.php

ISO 639-3

ISO 639-3 extends the ISO 639-2 alpha-3 codes with an aim to cover all known natural languages and works better for such languages as Cebuano, Montenegrin, Quechua (which has variations by region of the country) languages. For example, it’s recommended in the ALICIA repository Guide (also a video guide, Peru).   

Metadata recommendations for text material stored in Finnish publication repositories recommend the ISO 639-X standard for dc.language.iso. It is preferable to use the 3-character language codes of ISO 639-2 or ISO 639-3, as appropriate. 

There are still some implementation issues for a three-letter code as not all repositories could support this now (software and XML language issues) and there might be similar issues with aggregators (for example, OpenAIRE follows https://www.w3.org/TR/xml/ and https://www.w3.org/TR/xml/#RFC1766).  

More about language tags

A useful and more descriptive article on “Language tags in HTML and XML” (2014) from W3 with examples:

and a proposal to use

For many lesser-known languages spoken by minorities and also for historical stages of languages, language codes, the basis of language tags, are simply not available, see “The Shortcomings of Language Tags for Linked Data When Modeling Lesser-Known Languages” with recommendations to improve or develop ISO language codes. 

Source: https://comments.coar-repositories.org/appendix-4-iso-639-1-iso-639-2-and-iso-639-3-implementation-examples/