teidata.language defines the range of attribute values used to identify a particular combination of human language and writing system. [6.1. Language Identification]
Used by
Content model
  <dataRef name="language"/>
   <valItem ident=""/>
tei_teidata.language = xsd:language | ( "" )

The values for this attribute are language ‘tags’ as defined in BCP 47. Currently BCP 47 comprises RFC 5646 and RFC 4647; over time, other IETF documents may succeed these as the best current practice.

A ‘language tag’, per BCP 47, is assembled from a sequence of components or subtags separated by the hyphen character (-, U+002D). The tag is made of the following subtags, in the following order. Every subtag except the first is optional. If present, each occurs only once, except the fourth and fifth components (variant and extension), which are repeatable.

The IANA-registered code for the language. This is almost always the same as the ISO 639 2-letter language code if there is one. The list of available registered language subtags can be found at http://www.iana.org/assignments/language-subtag-registry. It is recommended that this code be written in lower case.
The ISO 15924 code for the script. These codes consist of 4 letters, and it is recommended they be written with an initial capital, the other three letters in lower case. The canonical list of codes is maintained by the Unicode Consortium, and is available at http://unicode.org/iso15924/iso15924-codes.html. The IETF recommends this code be omitted unless it is necessary to make a distinction you need.
Either an ISO 3166 country code or a UN M.49 region code that is registered with IANA (not all such codes are registered, e.g. UN codes for economic groupings or codes for countries for which there is already an ISO 3166 2-letter code are not registered). The former consist of 2 letters, and it is recommended they be written in upper case; the list of codes can be searched or browsed at https://www.iso.org/obp/ui/#search/code/. The latter consist of 3 digits; the list of codes can be found at http://unstats.un.org/unsd/methods/m49/m49.htm.
An IANA-registered variation. These codes ‘are used to indicate additional, well-recognized variations that define a language or its dialects that are not covered by other available subtags’.
An extension has the format of a single letter followed by a hyphen followed by additional subtags. These exist to allow for future extension to BCP 47, but as of this writing no such extensions are in use.
private use
An extension that uses the initial subtag of the single letter x (i.e., starts with x-) has no meaning except as negotiated among the parties involved. These should be used with great care, since they interfere with the interoperability that use of RFC 4646 is intended to promote. In order for a document that makes use of these subtags to be TEI-conformant, a corresponding language element must be present in the TEI header.

There are two exceptions to the above format. First, there are language tags in the IANA registry that do not match the above syntax, but are present because they have been ‘grandfathered’ from previous specifications.

Second, an entire language tag can consist of only a private use subtag. These tags start with x-, and do not need to follow any further rules established by the IETF and endorsed by these Guidelines. Like all language tags that make use of private use subtags, the language in question must be documented in a corresponding language element in the TEI header.

Examples include

Chinese written in traditional script as used in Hong Kong
English as spoken in Sierra Leone
Spanish as spoken in Mexico
Spanish as spoken in Latin America

The W3C Internationalization Activity has published a useful introduction to BCP 47, Language tags in HTML and XML.