Language tags that denote writing systems

[Jon Hanna]

> I don't see what the problem is converting yi-Latn to yi is; even if
> it's not entirely correct, it will get done anyway. Worry more about
> the incorrect xx-foo to xx conversions.

That *is* the problem with yi-Latn -> yi! yi-Latn -> yi is correct, but
there is no way to know that without recording the fact somewhere.
Since one cannot assume any hierarchical system within the tags (even the
case where a ISO 3166 and ISO 639 are combined gives us the totally separate
sgn-IE, sgn-GB, sgn-US etc.) and must treat them as opaque identifiers a
conversion that may be necessary (same language different script, or same
language and one isn't written) is going to be problematic.