@littledan this is a proposal we could work into our Locale spec, if we can get group to agree on the change.

Current spec (and most of the constructors) expect bcp-47 locale id. A cleaner approach would be to use Unicode locale ID, see here for differences:

http://unicode.org/repos/cldr/trunk/specs/ldml/tr35.html#BCP_47_Conformance

It does not allow for the full syntax of [BCP47]:

• No irregular or BCP47 grandfathered tags are allowed
• No extlang subtags are allowed
• A tag must not start with the subtag "x". Thus a privateuse (eg x-abc) can only be after a language subtag like "und"

It allows for certain additions:

• For field separator characters, the "_" character can be used as well as the "-" used in [BCP47].
• "root" to indicate the generic locale used as the parent of all languages in the CLDR data model.
• Certain codes that are private-use in BCP-47 and ISO are given semantics by LDML.
• Each macrolanguage has an identified primary encompassed language. That encompassed language is treated as an alias for the macrolanguage, and thus is replaced when canonicalizing.
• The language tag may begin with a script rather than a language (specialized use only).

There are multiple problems with bcp-47 tags, from slightly annoying grandfathered tags (source of most Locale bugs in v8), to script mapping.

For example:

• Canonicalization of zh, zh-cmn - https://unicode-org.atlassian.net/browse/ICU-7656
• ICU strict bcp-47 implementation (given migration to unicode language ids, team is hesitant to do this) - https://unicode-org.atlassian.net/browse/ICU-20167