📄 rfc3066.txt
字号:
Network Working Group H. Alvestrand
Request for Comments: 3066 Cisco Systems
BCP: 47 January 2001
Obsoletes: 1766
Category: Best Current Practice
Tags for the Identification of Languages
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes a language tag for use in cases where it is
desired to indicate the language used in an information object, how
to register values for use in this language tag, and a construct for
matching such language tags.
1. Introduction
Human beings on our planet have, past and present, used a number of
languages. There are many reasons why one would want to identify the
language used when presenting information.
In some contexts, it is possible to have information available in
more than one language, or it might be possible to provide tools
(such as dictionaries) to assist in the understanding of a language.
Also, many types of information processing require knowledge of the
language in which information is expressed in order for that process
to be performed on the information; for example spell-checking,
computer-synthesized speech, Braille, or high-quality print
renderings.
One means of indicating the language used is by labeling the
information content with an identifier for the language that is used
in this information content.
Alvestrand Best Current Practice [Page 1]
RFC 3066 Tags for Identification of Languages January 2001
This document specifies an identifier mechanism, a registration
function for values to be used with that identifier mechanism, and a
construct for matching against those values.
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
2. The Language tag
2.1 Language tag syntax
The language tag is composed of one or more parts: A primary language
subtag and a (possibly empty) series of subsequent subtags.
The syntax of this tag in ABNF [RFC 2234] is:
Language-Tag = Primary-subtag *( "-" Subtag )
Primary-subtag = 1*8ALPHA
Subtag = 1*8(ALPHA / DIGIT)
The productions ALPHA and DIGIT are imported from RFC 2234; they
denote respectively the characters A to Z in upper or lower case and
the digits from 0 to 9. The character "-" is HYPHEN-MINUS (ABNF:
%x2D).
All tags are to be treated as case insensitive; there exist
conventions for capitalization of some of them, but these should not
be taken to carry meaning. For instance, [ISO 3166] recommends that
country codes are capitalized (MN Mongolia), while [ISO 639]
recommends that language codes are written in lower case (mn
Mongolian).
2.2 Language tag sources
The namespace of language tags is administered by the Internet
Assigned Numbers Authority (IANA) [RFC 2860] according to the rules
in section 3 of this document.
The following rules apply to the primary subtag:
- All 2-letter subtags are interpreted according to assignments found
in ISO standard 639, "Code for the representation of names of
languages" [ISO 639], or assignments subsequently made by the ISO
639 part 1 maintenance agency or governing standardization bodies.
(Note: A revision is underway, and is expected to be released as
Alvestrand Best Current Practice [Page 2]
RFC 3066 Tags for Identification of Languages January 2001
ISO 639-1:2000)
- All 3-letter subtags are interpreted according to assignments found
in ISO 639 part 2, "Codes for the representation of names of
languages -- Part 2: Alpha-3 code [ISO 639-2]", or assignments
subsequently made by the ISO 639 part 2 maintenance agency or
governing standardization bodies.
- The value "i" is reserved for IANA-defined registrations
- The value "x" is reserved for private use. Subtags of "x" shall
not be registered by the IANA.
- Other values shall not be assigned except by revision of this
standard.
The reason for reserving all other tags is to be open towards new
revisions of ISO 639; the use of "i" and "x" is the minimum we can do
here to be able to extend the mechanism to meet our immediate
requirements.
The following rules apply to the second subtag:
- All 2-letter subtags are interpreted as ISO 3166 alpha-2 country
codes from [ISO 3166], or subsequently assigned by the ISO 3166
maintenance agency or governing standardization bodies, denoting
the area to which this language variant relates.
- Tags with second subtags of 3 to 8 letters may be registered with
IANA, according to the rules in chapter 5 of this document.
- Tags with 1-letter second subtags may not be assigned except after
revision of this standard.
There are no rules apart from the syntactic ones for the third and
subsequent subtags.
Tags constructed wholly from the codes that are assigned
interpretations by this chapter do not need to be registered with
IANA before use.
The information in a subtag may for instance be:
- Country identification, such as en-US (this usage is described in
ISO 639)
- Dialect or variant information, such as en-scouse
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RFC 3066 Tags for Identification of Languages January 2001
- Languages not listed in ISO 639 that are not variants of any listed
language, which can be registered with the i-prefix, such as i-
tsolyani
- Region identification, such as sgn-US-MA (Martha's Vineyard Sign
Language, which is found in the state of Massachusetts, US)
This document leaves the decision on what tags are appropriate or not
to the registration process described in section 3.
ISO 639 defines a maintenance agency for additions to and changes in
the list of languages in ISO 639. This agency is:
International Information Centre for Terminology (Infoterm)
P.O. Box 130
A-1021 Wien
Austria
Phone: +43 1 26 75 35 Ext. 312
Fax: +43 1 216 32 72
ISO 639-2 defines a maintenance agency for additions to and changes
in the list of languages in ISO 639-2. This agency is:
Library of Congress
Network Development and MARC Standards Office
Washington, D.C. 20540
USA
Phone: +1 202 707 6237
Fax: +1 202 707 0115
URL: http://www.loc.gov/standards/iso639
The maintenance agency for ISO 3166 (country codes) is:
ISO 3166 Maintenance Agency Secretariat
c/o DIN Deutsches Institut fuer Normung
Burggrafenstrasse 6
Postfach 1107
D-10787 Berlin
Germany
Phone: +49 30 26 01 320
Fax: +49 30 26 01 231
URL: http://www.din.de/gremien/nas/nabd/iso3166ma/
ISO 3166 reserves the country codes AA, QM-QZ, XA-XZ and ZZ as user-
assigned codes. These MUST NOT be used to form language tags.
Alvestrand Best Current Practice [Page 4]
RFC 3066 Tags for Identification of Languages January 2001
2.3 Choice of language tag
One may occasionally be faced with several possible tags for the same
body of text.
Interoperability is best served if all users send the same tag, and
use the same tag for the same language for all documents. If an
application has requirements that make the rules here inapplicable,
the application protocol specification MUST specify how the procedure
varies from the one given here.
The text below is based on the set of tags known to the tagging
entity.
1. Use the most precise tagging known to the sender that can be
ascertained and is useful within the application context.
2. When a language has both an ISO 639-1 2-character code and an ISO
639-2 3-character code, you MUST use the tag derived from the ISO
639-1 2-character code.
3. When a language has no ISO 639-1 2-character code, and the ISO
639-2/T (Terminology) code and the ISO 639-2/B (Bibliographic)
code differ, you MUST use the Terminology code. NOTE: At present,
all languages for which there is a difference have 2-character
codes, and the displeasure of developers about the existence of 2
code sets has been adequately communicated to ISO. So this
situation will hopefully not arise.
4. When a language has both an IANA-registered tag (i-something) and
a tag derived from an ISO registered code, you MUST use the ISO
tag. NOTE: When such a situation is discovered, the IANA-
registered tag SHOULD be deprecated as soon as possible.
5. You SHOULD NOT use the UND (Undetermined) code unless the protocol
in use forces you to give a value for the language tag, even if
the language is unknown. Omitting the tag is preferred.
6. You SHOULD NOT use the MUL (Multiple) tag if the protocol allows
you to use multiple languages, as is the case for the Content-
Language: header.
NOTE: In order to avoid versioning difficulties in applications such
as that of RFC 1766, the ISO 639 Registration Authority Joint
Advisory Committee (RA-JAC) has agreed on the following policy
statement:
Alvestrand Best Current Practice [Page 5]
RFC 3066 Tags for Identification of Languages January 2001
"After the publication of ISO/DIS 639-1 as an International
Standard, no new 2-letter code shall be added to ISO 639-1 unless a
3-letter code is also added at the same time to ISO 639-2. In
addition, no language with a 3-letter code available at the time of
publication of ISO 639-1 which at that time had no 2-letter code
shall be subsequently given a 2-letter code."
This will ensure that, for example, a user who implements "hwi"
(Hawaiian), which currently has no 2-letter code, will not find his
or her data invalidated by eventual addition of a 2-letter code for
that language."
2.4 Meaning of the language tag
The language tag always defines a language as spoken (or written,
signed or otherwise signaled) by human beings for communication of
information to other human beings. Computer languages such as
programming languages are explicitly excluded. There is no
guaranteed relationship between languages whose tags begin with the
same series of subtags; specifically, they are NOT guaranteed to be
mutually intelligible, although it will sometimes be the case that
they are.
The relationship between the tag and the information it relates to is
defined by the standard describing the context in which it appears.
Accordingly, this section can only give possible examples of its
usage.
- For a single information object, it could be taken as the set of
languages that is required for a complete comprehension of the
complete object.
Example: Plain text documents.
- For an aggregation of information objects, it should be taken as
the set of languages used inside components of that aggregation.
Examples: Document stores and libraries.
- For information objects whose purpose is to provide alternatives,
the set of tags associated with it should be regarded as a hint
that the content is provided in several languages, and that one has
to inspect each of the alternatives in order to find its language
or languages. In this case, a tag with multiple languages does not
mean that one needs to be multi-lingual to get complete
understanding of the document.
Example: MIME multipart/alternative.
Alvestrand Best Current Practice [Page 6]
RFC 3066 Tags for Identification of Languages January 2001
- In markup languages, such as HTML and XML, language information can
be added to each part of the document identified by the markup
structure (including the whole document itself). For example, one
could write <span lang="FR">C'est la vie.</span> inside a Norwegian
document; the Norwegian-speaking user could then access a French-
Norwegian dictionary to find out what the marked section meant. If
the user were listening to that document through a speech synthesis
interface, this formation could be used to signal the synthesizer
to appropriately apply French text-to-speech pronunciation rules to
that span of text, instead of misapplying the Norwegian rules.
2.5 Language-range
Since the publication of RFC 1766, it has become apparent that there
is a need to define a term for a set of languages whose tags all
begin with the same sequence of subtags.
The following definition of language-range is derived from HTTP/1.1
[RFC 2616].
language-range = language-tag / "*"
That is, a language-range has the same syntax as a language-tag, or
is the single character "*".
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