Locale codes of CSL locale files

Hi all,

After discussing it with Frank, I decided to change the values of xml:lang
on cs:locale in the CSL 1.0 locale styles to the “xx-XX” locale code in
their file names. Before this change the values consisted only of the
language code (“xx”), which was a bit weird (e.g. locales-pt-BR.xml and
locales-pt-PT.xml both had an xml:lang value of “pt”).

According to Frank, this change shouldn’t affect citeproc-js, but I’d like
to mention it for other implementors. In related news, I just added an en-GB
locale with UK dates and grammar settings.