Added ISO 639-1 2-letter language code support.

Added ISO 639-1 2-letter language code support.
Note: there has been a change in the draft Recommendation,
such that <language> is now defined as either an ISO 639-2
3-letter code or an ISO 639-1 2-letter code.  As RFC 3066,
to which xml:lang values are supposed to conform, expresses
a preference for ISO 639-1, and Java Locales are built from
the 2-letter codes, this was necessary.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/branches/FOP_0-20-0_Alt-Design@197659 13f79535-47bb-0310-9956-ffa450edef68