Non-English language support via polyglossia.

Add xunicode and polyglossia packages for better language support.
Process HTML 'lang' attributes and emit the proper polyglossia
language-change command.  Add texlive-lang-all package to get all the
appropriate hyphenation patterns, etc.

Set default font to GNU freefont's "FreeSerif" face, which has better
non-latin support (but no CJK characters).

Change-Id: I08454de9c33e5971c76db2313b4bc2a48e74af1d