One document matched: draft-alvestrand-lang-tag-v2-02.txt
Differences from draft-alvestrand-lang-tag-v2-01.txt
Internet-Draft H. Alvestrand
draft-alvestrand-lang-tag-v2-02.txt EDB Maxware
Target Category: Standards Track
June 2000
Obsoletes: RFC 1766 Expires: December 2000
Tags for the Identification of Languages
Status of this Memo
The file name of this memo is draft-alvestrand-lang-tag-v2-0.txt
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as "work
in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Comments on this draft should be sent to the mailing list <ietf-
languages@iana.org>
Abstract
This document describes a language tag for use in cases where it is
desired to indicate the language used in an information object.
It also defines a "Content-language:" header, for use in the case where
one desires to indicate the language of something that has RFC-822-like
headers, like MIME body parts or Web documents, and a new parameter to
the Multipart/Alternative type, to aid in the usage of the Content-
Language: header.
Tags for the names of languages Harald Alvestrand
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1. Introduction
There are a number of languages presently or previously used by human
beings in this world.
A great number of these people would prefer to have information
presented in a language which they understand.
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.
In other cases, it may be desirable to use a computer program to
convert information from one format (such as plaintext) into another
(such as computer-synthesized speech, or Braille, or high-quality print
renderings).
A prerequisite for any such function is a means of labelling the
information content with an identifier for the language that is used in
this information content.
This document specifies an identifier mechanism, and one possible use
for it.
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
tag and a (possibly empty) series of subtags.
The syntax of this tag in RFC 2234 ABNF is:
Language-Tag = Primary-tag *( "-" Subtag )
Primary-tag = 1*8ALPHA
Subtag = 1*8ALPHA
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).
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2.2 Language tag sources
The namespace of language tags is administered by the IANA according to
the rules in section 5 of this document.
The following registrations are predefined:
In the primary language tag:
- All 2-letter tags are interpreted according to assignments found in
ISO standard 639, "Code for the representation of names of languages"
[ISO 639], or subsequently made by the standardÆs registration
authority.
(Note: A revision is underway, and is expected to be released as ISO
639-1:2000)
- All 3-letter tags 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 subsequently made by the
standardÆs registration authority.
- 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.
In the first subtag:
- All 2-letter codes are interpreted as ISO 3166 alpha-2 country codes
denoting the area in which the language is used.
- Codes of 3 to 8 letters may be registered with the IANA, according to
the rules in chapter 5 of this document.
The information in the subtag may for instance be:
- Country identification, such as en-US (this usage is described in ISO
639)
- Dialect or variant information, such as no-nyn (nynorsk) or en-scouse
- 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-
cherokee
- Script variations, such as az-Arab and az-Cyrl (Azerbaijani in Arabic
or Cyrillic script - these script codes are suggested by the pending
script code standard ISO/DIS 15924)
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This document does not place any restriction on what values one can
register here, as long as they conform to the rules in section 5.
NOTE IN DRAFT: It has been suggested that subtags of 4 characters
be reserved for ISO/DIS 15924 codes. Opinions for and against are
sought.
ISO 639 defines a registration authority for additions to and changes
in the list of languages in ISO 639. This authority 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 registration authority for additions to and changes
in the list of languages in ISO 639-2. This authority 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 registration 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
ISO 3166 reserves the country codes AA, QM-QZ, XA-XZ and ZZ as user-
assigned codes.
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. Exact
requirements may need to vary by application area; if so, the
application protocol specification MUST specify how the procedure
varies from the one given here.
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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.
2. When a language has both an ISO 639-1 2-character tag and an ISO 639-
2 3-character tag, you MUST use the ISO 639-1 2-character tag.
3. When a language has no ISO 639-1 2-character tag, and the ISO 639-2/T
(Terminology) tag and the ISO 639-2/B (Bibliographic) tag differ, you
MUST use the Terminology tag.
NOTE: At present, all languages for which there is a difference have
2-character tags, and the displeasure of developers about the
existence of 2 tag 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 an
ISO registered tag, 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) tag 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 MUST 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 RA-JAC has agreed on the following policy
statement:
"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"
(HawaiÆian), 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 signalled) by human beings for communication of
information to other human beings.
Computer languages such as programming languages are explicitly
excluded.
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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.
Applications should always treat a language tag as a single token; the
division into main tag and subtags is an administrative mechanism, not
a navigation aid.
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 should 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, it
should be regarded as a hint that the material inside 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, multiple languages need not mean that one needs to be
multilingual to get complete understanding of the document.
Example: MIME multipart/alternative.
- In markup languages, such as HTML, it is possible to define a
construct embedding a language tag to indicate that contained text is
written in this language, such that one could write <DIV
lang="FR">C'est la vie</DIV> 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 that share some common
property. The following definition of language-range is derived from
RFC 2616 (HTTP/1.1).
language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) / "*" )
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A language-range matches a language-tag if it exactly equals the tag,
or if it exactly equals a prefix of the tag such that the first tag
character following the prefix is "-".
The special range "*" matches any tag. A protocol which uses language
ranges may specify additional rules about the semantics of "*"; for
instance, HTTP/1.1 specifies that it only matches languages not matched
by any other range within an "Accept-Language:" header.
NOTE: This use of a prefix matching rule does not imply that language
tags are assigned to languages in such a way that it is always true
that if a user understands a language with a certain tag, then this
user will also understand all languages with tags for which this tag is
a prefix. The prefix rule simply allows the use of prefix tags if this
is the case.
3. The Content-language header
The "Content-Language" header is intended for use in the case where one
desires to indicate the language(s) of something that has RFC-822-like
headers, such as MIME body parts or Web documents.
The RFC-822 EBNF of the Content-Language header is:
Content-Language = "Content-Language" ":" 1#Language-tag
Or in RFC 2234 ABNF:
Content-Language = "Content-Language" CFWS ":" Language-List
Language-List = Language-Tag [ CFWS "," CFWS Language-List ]
The Content-Language header may list several languages in a comma-
separated list.
The CFWS construct is intended to function like the whitespace
convention in RFC 822, which means also that one can place
parenthesized comments anywhere in the language sequence, or use
continuation lines. A formal definition is given in the update to the
RFC 822 grammar, currently a work in progress.
3.1 Examples of Content-language values
Norwegian official document, with parallel text in both official
versions of Norwegian. (Both versions are readable by all Norwegians).
Content-Type: multipart/alternative;
differences=content-language
Content-Language: no-nyn, no-bok
Voice recording from Liverpool downtown
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Content-type: audio/basic
Content-Language: en-scouse
Document in Mingo, an American Indian language which does not have an
ISO 639 code:
Content-type: text/plain
Content-Language: i-mingo
An English-French dictionary
Content-type: application/dictionary
Content-Language: en, fr (This is a dictionary)
An official European Commission document (in a few of its official
languages)
Content-type: multipart/alternative
Content-Language: da, de, el, en, fr, it
An excerpt from Star Trek
Content-type: video/mpeg
Content-Language: i-klingon
(All the tags used in these examples were registered with IANA after
the publication of RFC 1766)
4. IANA registration procedure for language tags
Any language tag shall begin with an existing tag, and extend it.
The registration form given here must be used by anyone who wants to
use a language tag not defined by ISO or IANA.
----------------------------------------------------------------------
LANGUAGE TAG REGISTRATION FORM
Name of requester :
E-mail address of requester:
Tag to be registered :
English name of language :
Native name of language (transcribed into ASCII):
Reference to published description of the language (book or article):
Any other relevant information:
----------------------------------------------------------------------
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The language form must be sent to <ietf-languages@iana.org> for a 2-
week review period before it can be submitted to IANA. (This is an
open list. Requests to be added should be sent to <ietf-languages-
request@iana.org>.)
When the two week period has passed, the language tag reviewer, who is
appointed by the IETF Applications Area Director, either forwards the
request to IANA@ISI.EDU, or rejects it because of significant
objections raised on the list. Note that the reviewer can raise
objections on the list himself, if he so desires. The important thing
is that the objection must be made publicly.
The applicant is free to modify a rejected application with additional
information and submit it again; this restarts the 2-week comment
period.
Decisions made by the reviewer may be appealed to the IESG.
All registered forms are available online in the directory
ftp://ftp.isi.edu/in-notes/iana/assignments/languages/
Updates of registrations follow the same procedure as registrations.
The language tag reviewer decides whether to allow a new registrant to
update a registration made by someone else; in the normal case,
objections by the original registrant would carry extra weight in such
a decision.
There is no deletion of registrations; when some registered tag should
not be used any more, for instance because a corresponding ISO 639 code
has been registered, the registration should be amended by adding a
remark like "DO NOT USE: use <new code> instead" to the "other relevant
information" section.
5. Security Considerations
The only security issue that has been raised with language tags since
the publication of RFC 1766, which stated that "Security issues are
believed to be irrelevant to this memo", is a concern with language
ranges used in content negotiation - that they may be used to infer the
nationality of the sender, and thus identify potential targets for
surveilllance.
This is a special case of the general problem that anything you send is
visible to the receiving party; it is useful to be aware that such
concerns can exist in some cases.
The exact magnitude of the threat, and any possible countermeasures, is
left to each application protocol.
6. Character set considerations
Codes may always be expressed using the US-ASCII character repertoire
(a-z), which is present in most character sets.
The issue of deciding upon the rendering of a character set based on
the language tag is not addressed in this memo; however, it is thought
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impossible to make such a decision correctly for all cases unless means
of switching language in the middle of a text are defined (for example,
a rendering engine that decides font based on Japanese or Chinese
language may fail to work when a mixed Japanese-Chinese text is
encountered)
7. Acknowledgements
This document has benefited from many rounds of review and comments in
various fora of the IETF and the Internet working groups.
Any list of contributors is bound to be incomplete; please regard the
following as only a selection from the group of people who have
contributed to make this document what it is today.
In alphabetical order:
Tim Berners-Lee, Nathaniel Borenstein, Sean M. Burke, Jim Conklin, John
Cowan, Dave Crocker, Martin Duerst, Michael Everson, Ned Freed, Tim
Goodwin, Dirk-Willem van Gulik, Paul Hoffman, Olle Jarnefors, John
Klensin, Keith Moore, Masataka Ohta, Keld Jorn Simonsen, Rhys
Weatherley, Misha Wolf, Francois Yergeau and many, many others.
Special thanks must go to Michael Everson, who has served as language
tag reviewer for almost the complete period since the publication of
RFC 1766, and has provided a great deal of input to this revision.
8. Author's Address
Harald Tveit Alvestrand
EDB Maxware
Pirsenteret
N-7462 TRONDHEIM
NORWAY
EMail: Harald.Alvestrand@maxware.no
Phone: +47 73 54 57 97
9. References
[ISO 639]
ISO 639:1988 (E/F) - Code for the representation of names of
languages - The International Organization for Standardization,
1st edition, 1988-04-01 Prepared by ISO/TC 37 - Terminology
(principles and coordination).
Note that a new version (ISO 639-1:2000) is in preparation at the
time of this writing.
[ISO 639-2]
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ISO 639-2:1998 - Codes for the representation of names of
languages -- Part 2: Alpha-3 code - edition 1, 1998-11-01, 66
pages, prepared by ISO/TC 37/SC 2
[ISO 3166]
ISO 3166:1988 (E/F) - Codes for the representation of names of
countries - The International Organization for Standardization,
3rd edition, 1988-08-15.
[ISO 15924]
ISO/DIS 15924 - Codes for the representation of names of scripts
(under development by ISO TC46/SC2)
[RFC 1327]
Kille, S., "Mapping between X.400(1988) / ISO 10021 and RFC 822",
RFC 1327, University College London, May 1992.
[RFC 1521]
Borenstein, N., and N. Freed, "MIME Part One: Mechanisms for
Specifying and Describing the Format of Internet Message Bodies",
RFC 1521, Bellcore, Innosoft, September 1993.
[RFC 2119]
Key words for use in RFCs to Indicate Requirement Levels. S.
Bradner. March 1997.
[RFC 2234]
Augmented BNF for Syntax Specifications: ABNF. D. Crocker, Ed., P.
Overell, November 1997.
[RFC 2616]
Hypertext Transfer Protocol -- HTTP/1.1. R. Fielding, J. Gettys,
J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee. June
1999.
Appendix A: Language Tag Reference Material
The Library of Congress, maintainers of ISO 639-2, has made the list of
languages registered available on the Internet.
At the time of this writing, it can be found at
http://www.loc.gov/standards/iso639-2/langhome.html
The IANA registration forms for registered language codes can be found
at
http://www.isi.edu/in-notes/iana/assignments/languages/
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Appendix B: Changes from RFC 1766
. Email list address changed from ietf-types@uninett.no to ietf-
languages@iana.org
. Updated author's address
. Added language-range construct from HTTP/1.1
. Added use of ISO 639-2 language codes
. Added reference to Library of Congress lists of language codes
. Changed examples to use registered tags
. Moved Multipart/Alternative-related stuff to appendix C
. Added "Any other information" to registration form
. Added description of procedure for updating registrations
Appendix C: Use of Content-Language with Multipart/Alternative
NOTE: This appendix details an idea that was proposed in RFC 1766 to
deal with a particular kind of alternative content. However, this has
not found use in practice, and is therefore not suitable for the IETF
standards track. It is being preserved here as a non-normative appendix
only.
When using the Multipart/Alternative body part of MIME, it is possible
to have the body parts giving the same information content in different
languages. In this case, one should put a Content-Language header on
each of the body parts, and a summary Content-Language header onto the
Multipart/Alternative itself.
The differences parameter to multipart/alternative
As defined in RFC 1541, "Multipart/Alternative" only has one parameter:
boundary.
The common usage of "Multipart/Alternative" is to have more than one
format of the same message (f.ex. PostScript and ASCII).
The use of language tags to differentiate between different
alternatives will certainly not lead all MIME UAs to present the most
meaningful, understandable or significant body part as default.
Therefore, a new parameter is defined, to allow the configuration of
MIME readers to handle language differences in a sensible manner.
Name: Differences
Value: One or more of
Content-Type
Content-Language
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Further values can be registered with IANA; these shall refer to the
name of a header for which a definition exists in a published RFC. If
not present, "Differences=Content-Type" is assumed.
The intent is that the MIME reader can look at these headers of the
message component to make an intelligent choice of what to present to
the user, based on knowledge about the user preferences and
capabilities.
(The intent of having registration with IANA of the fields used in this
context is to maintain a list of usages that a mail UA may expect to
encounter, not to reject usages.)
(NOTE: The MIME specification [RFC 1521], section 7.2, states that
headers not beginning with "Content-" are generally to be ignored in
body parts. People defining a header for use with "differences=" should
take note of this.)
The mechanism for deciding which body part to present is outside the
scope of this document.
MIME EXAMPLE:
Content-Type: multipart/alternative; differences=Content-Language;
boundary="limit"
Content-Language: en, fr, de
--limit
Content-Language: fr
Le renard brun et agile saute par dessus le chien paresseux
--limit
Content-Language: de
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-encoding: quoted-printable
Der schnelle braune Fuchs h=FCpft =FCber den faulen Hund
--limit
Content-Language: en
The quick brown fox jumps over the lazy dog
--limit--
When composing a message, the choice of sequence may be arbitrary.
However, non-MIME mail readers will show the first body part first,
meaning that this should most likely be the language understood by most
of the recipients.
Appendix X1: Changes from draft -00 to -01
This appendix is to be deleted by the RFC Editor before publication as
RFC.
Changes from draft-00:
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- Fixed up the language tag table
- Moved multipart/alternative stuff to appendix
- Changed examples to use registered tags
- Added * in languagte tag table to indicate B/T conflicts
- Considered, but did not adopt, changing from recommending T codes to
recommending B codes. At the moment, the only argument that appeals
to the author is that the T codes look more like the 639-1 codes than
the B codes do.
- Added procedures for updating a registration
Here is the list of changes that need to be done to this doc before
advancing it to Draft or reissuing it.
- Decide whether or not to write anything about use of country codes in
other places than the first subtag, or region codes, or script codes
- Decide whether it is worth it to try to write down any more
guidelines for what language tags people should register
Appendix X2: Changes from draft -01 to -02
This appendix is to be deleted by the RFC Editor before publication as
RFC.
- Minor updates
- Added reference to Library of Congress code lists instead of
including code values
- Changed grammars to use RFC 2234 ABNF
- Used MUST and SHOULD in label choice algorithm
TODO list
Consider whether to include Accept-Language: as a generic. If so,
decide whether to use the HTTP standard's "weight factor" or the
nonstandard, but commonly used "best language first".
Suggested language, to be placed as section 3.2:
The "Accept-Language" header is intended for use in the case where
a user or a process
desires to indentify the language(s) he prefers when RFC-822-like
headers, such as MIME body parts or Web documents are used.
The RFC-822 EBNF of the Accept-Language header is:
Accept-Language = "Accept-Language" ":" Language-List
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The Accept-Language header may list several languages in a
comma-separated list. The leftmost is the most preferred.
Note that HTTP uses the same header name with a different syntax;
see RFC 2616 for the details.
Consider whether more guidance on "appropriate" tags is needed.
Consider whether we need to allow numbers in language tags.
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