Write reliable information about the publication language #8
Description
This commit blindly assigns the value 'en' to the DC:Language tag.
This is fair, since the vast majority of our publications are written in English. However, it would be desirable to pull this information from a reliable source (crossref at present, tooth in the near future). I haven't found a (proper) way to archive this result with crossref; let's have this fixed when we modify chapter splitter to work with tooth.