What do you think about the definition “It can also be used to indicate that content is expressed in a way that is appropriate for use throughout a region, for instance, Spanish content tailored to be useful throughout Latin America.”? Is that about language only? BCP 47 has been extended to certainly much more than languages now, if you insist otherwise.
- “The main difference among locales is in terms of language; there may also be some differences according to different countries or regions. However, the line between locales and languages, as commonly used in the industry, are rather fuzzy.” Unicode Locale Data Markup Language (LDML)
- " In practice, many people use [BCP47] codes to mean locale codes instead of strictly language codes. It is easy to see why this came about; because [BCP47] includes an explicit region (territory) code, for most people it was sufficient for use as a locale code as well. For example, when typical web software receives a [BCP47] code, it will use it as a locale code. Other typical software will do the same: in practice, language codes and locale codes are treated interchangeably." Unicode Locale Data Markup Language (LDML)
Content for a certain country in that language, and content in that language of a certain country, are practically interchangable. This is further clarified by different *_name= in OSM. If you need an exact distinction, it should be a special case, and handled specially. There’s no need to use eg official_name:en-US:US= to mean content for US in English in US by default.
Regardless, for other languages in USA, you could ask about adding other official_name:*-US afterwards. Or perhaps more precisely if it is considered that there is no official US language, or there’s no official decision in other langauges, technically official_name:*-US-t-en-US= as I pointed towards?