[osis-core] warning on use of xs:language & xml:lang

[Chris Little]

I tried to validate a document that includes an ISO 639-2 value in an 
xml:lang attribute and found that they are invalid.  The XML Schema WG 
knows about the problem and records it in the errata: 
http://www.w3.org/2001/05/xmlschema-errata#e2-25 .

The problem stems partly from the update from RFC 1766 to RFC 3066, 
which allows 3-letter codes.  But the greater problem is that the regex 
for xs:language in XML Schema 1.0 was never a correct reflection of the 
pattern represented in either RFC.

This means we cannot correctly identify languages such as Aramaic (arc) 
or Ancient Greek (grc) as RFC 3066 directs.

Perhaps we could recommend using "x-iso-[code]" (e.g. x-iso-grc for 
Ancient Greek) until Schema is updated.

--Chris