Language-Tagged Strings (SUPERSEDED)

This document is obsolete and has been superseded by Appendix A of RFC 9290, "Concise Problem Details for Constrained Application Protocol (CoAP) APIs", October 2022.

This document registers a tag for serializing language-tagged strings in Concise Binary Object Representation (CBOR) (ref. 1).


In some cases it's useful to specify the natural language of a text string. This specification defines a new tag that does just that. One technology that supports language-tagged strings is the Resource Description Framework (RDF) (ref. 2).

Detailed Semantics

A language-tagged string in CBOR has the tag 38 and consists of an array with a length of 2. The first element is a well-formed language tag under Best Current Practice 47 (ref. 3 and 4), represented as a UTF-8 text string (major type 3). The second element is an arbitrary UTF-8 text string (major type 3). Both the language tag and the arbitrary string can optionally be annotated with CBOR tags.

NOTE: Language tags of any combination of case are allowed. But section 2.1.1 of RFC 5646 (ref. 3), part of Best Current Practice 47, recommends a case combination, for language tags, that encoders that support tag 38 may wish to follow when generating language tags.

A CBOR decoder can treat data items with tag 38 that don't meet the criteria above as an error, but this specification doesn't define how a CBOR implementation ought to behave in this case. Section 3.4 of RFC 7049 (ref. 1) details this kind of error-handling behavior.

NOTE: The Unicode Standard (ref. 5) includes a set of characters designed for tagging text (including language tagging), in the range U+E0000 to U+E007F. Although many applications, including RDF (ref. 2), do not disallow these characters in text strings, the Unicode Consortium has deprecated these characters and recommends annotating language via a higher-level protocol instead. See the section "Deprecated Tag Characters" in the Unicode Standard (found at section 16.9 in version 6.2 of the Core Specification, the latest specification available at the time of this writing).


The following example shows how the English-language string "Hello" is encoded.

    d8 26      ---- Tag 38
       82      ---- Array length 2
          62 65 6E   ---- "en"
          65 48 65 6C 6C 6F ---- "Hello"

The following example shows how the French-language string "Bonjour" is encoded.

    d8 26     ---- Tag 38
       82      ---- Array length 2
          62 66 72   ---- "fr"
          67 42 6F 6E 6A 6F 75 72 ---- "Bonjour"


Ref. 1. Bormann, C. and Hoffman, P. "Concise Binary Object Representation (CBOR)". RFC 7049, October 2013.

Ref. 2 (informative). Cyganiak, R. et al. "RDF 1.1 Concepts and Abstract Syntax". W3C Recommendation, 25 Feb. 2014. <>

Ref. 3. Phillips, A. and Davis, M. "Tags for Identifying Languages". BCP 47, RFC 5646, September 2009.

Ref. 4. Phillips, A. and Davis, M. "Matching of Language Tags". BCP 47, RFC 4647, September 2006.

Ref. 5. The Unicode Consortium. "The Unicode Standard". <>


Peter Occil (poccil14 at gmail dot com)

My CBOR home page.

Any copyright to this specification is released to the Public Domain.


Carsten Bormann reviewed this document and gave helpful suggestions. John Cowan and Doug Ewell are also to be acknowledged. This document was also discussed in the "apps-discuss at" and "ltru at" mailing lists.