title_body: looking for an easy way to look up possible values for Dublin Core Metadata Element Set etc
upvoted_answer: Sigil (freeware) will automatically insert the proper metadata entries for epub3 (and epub2) books.
  
  (You'll have to delete the default dc:language en entry. 
  Then click Add Metadata, OK and select the language.)
  
  AFAIK, it supports all metadata entries listed in the epub3 and epub2 specs.