[osis-core] Schema: type on language

[Chris Little]

 From the manual:

6.3.6. language
A language element must be provided for each language used substantially 
in a work. The language may be specified using an ISO 639 or ISO 639-2, 
or SIL Ethnologue codes. The type attribute must be set to "ISO-639," 
"ISO-639-2," or "SIL," respectively. In the rare case that none of these 
is sufficient, a prose description should be inserted in the element and 
the type attribute set to "other".

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With the 1.9 schema, this is false and all of these values for type in 
the <language> tag are invalid.  But it seems like a good system.

A couple of modifications: ISO-639 actually refers to both ISO-639-1 and 
-2, so "ISO-639" should probably be changed to "ISO-639-1".  "SIL" 
should probably be changed to "Ethnologue", since that's reference work 
and SIL is essentially just its publisher.  (The Egyptian Grammar 
example uses this value, actually.) And a "LINGUIST" option should be 
added since the LINGUIST List picks up the slack where the Ethnologue 
leaves off.

I also think <language> should receive a role attribute like we added to 
<date>.  (Consider <language type="ISO-639-2" 
role="original">grc</language><language type="ISO-639-1" 
role="translation">en</language>.)

Some possible values for language's role include: original, translation, 
interlinear, quotation (for excerpts), didactic (for grammars), source & 
target (for dictionaries).

--Chris