The output of apt-cache provide a field named "Description Language:" in about every file, and - as far as I was able to find out - they all are empty. From man 8 apt-cache
For the specific meaning of the remainder of the output it is best to consult the apt source code
As I'm not able to use interpret the source myself:
Question: What would be a possible string / value / … for this field?
The "Description Language" field contains the language code for any available translation you have downloaded. These are stored in ...Translation-lang files in /var/lib/apt/lists (where lang is the language code). apt-get downloads the appropriate Translation files for the language it's run in; thus for example
sudo LANG=fr_FR.UTF-8 apt-get update
will download French translation files.
On my system apt-cache showpkg dpkg gives one empty entry (corresponding to the original description in the Packages files) and one en entry (corresponding to the "translation" in the Translation-en files).
Many packages have descriptions translated into other languages; see for example the French description of dpkg.
