For the triple of
language/country/script to match usefully in the general case by
RFC 3066 parsers (which are unaware of script in general), the first
and second subtags would have to remain language code and country
If you consider realistic scenarios, this makes the wrong assumption that
country distinctions generally matter more to users.
If you want to consider realistic scenarios, it is often the case that country
information is readily available as input to matching algorithms, whereas
script information is not. Therefore the assumption that scripts matter more
than countries may be true but isn't relevant.
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