Add parameter to specify the language

Docbook-xsl has already translated some keywords to Chinese.  Now they
will show up.

Makefile

@@ -40,6 +40,7 @@ book: validate profile-html
       --stringparam chunk.quietly $(CHUNK_QUIET) \
       --stringparam rootid "$(ROOT_ID)"          \
       --stringparam base.dir $(BASEDIR)/         \
+      --stringparam l10n.gentext.default.language "zh_cn" \
       stylesheets/lfs-chunked.xsl                \
       $(RENDERTMP)/lfs-html.xml