IETF language code
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IETF language codes are defined by BCP 47 currently RFC 4646 which was published September 2006. This RFC obsoleted RFC 3066, which in January 2001 obsoleted RFC 1766 from March 1995.
RFC 4646 uses ISO 639-1, ISO 639-2, ISO 3166-1 alpha-2, ISO 15924 and UN M.49 to define a language tag. Each language tag is composed of "subtags" which are not case sensitive and can also be empty.
The order of the subtags if not empty is language-script-region-variant-extension-private use.
It is written that subtags are separated from each other by a hyphen. This is not true for the given examples in cases where the subtags are empty. The example is en not en-----.
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[edit] Relation to other standards
The IETF only derives their subtags from ISO standards, they are therefore not ISO conform.
It also reserves some tag parts that currently do not exist. Section 2.2.1, point 4 reads:
- 4. All four-character language subtags are reserved for possible future standardization.
At the same time ISO 15924 is a four-character subtag, all ready.
[edit] Relation to ISO 639-3
There is also reference to ISO 639-3. Section 8:
- ...anticipates features of ISO 639-3 with the addition of the extended language subtags, as well as the possibility of other ISO 639 parts becoming useful for the formation of language tags in the future.
Section "2.2.2. Extended Language Subtags" defines these subtages. Extended language subtags MUST NOT be registered or used to form language tags. This implies that by users of this RFC the ISO 639-1 codes cannot be phased out.
[edit] Issues with ISO 3166-1
Since the ISO 3166-1 alpha-2 can change from time to time there is ambiguity in the use. E.g. CS could refer to Serbia and Montenegro or to Czechoslovakia.
Section 2.2.4. point 3 C&D solve this. [1] For ambiguous ISO 3166-1 codes the UN M.49 code shall be used.
ISO 3166-1 codes can also be marked as depreceated:
%% Type: region Subtag: TP Description: East Timor Added: 2005-10-16 Preferred-Value: TL Deprecated: 2002-11-15 %%
