In #450 we are considering the addition of a contextual expression attribute for the locale. Currently this says that the values must be a BCP47 language tag (or comma-separated sequence of tags, e.g. a language priority list). We should agree on the level of validation for conformance.

Generally speaking, we should probably require well-formed tags (according to BCP47's understanding of well-formed), at least at the syntax level.
We could require that the tags be valid (this requires checking that the subtags are in the registry)
We could require that the tags be valid Unicode Locale Identifiers (ULIs) (which has further canonicalization requirements)

Writing this issue to provide a separate place to discuss.