[bp-i18n-specdev] Consider combining defining language value checklist rules

stevenatkin has just created a new issue for 
https://github.com/w3c/bp-i18n-specdev:

== Consider combining defining language value checklist rules ==
I would possibly combine some rules into the following:

Values for language declarations must use "well formed" BCP 47 
language tags and must use BCP 47 for language tag matching.

Refer to BCP 47, not to RFC 5646.

See https://github.com/w3c/bp-i18n-specdev/issues/6
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