How to language tag language tags?

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If I wanted to create a HTML version of the language subtag registry, 
then I would tag the entire registry with lang="en" (<html lang="en">), 
while each entr in the registry perhaps could look like this:

<hr/>
<p>Type: language<br/>
   Subtag: <dfn lang="zxx">aa</dfn><br/>
   Description: Afar<br/>
   Added: 2005-10-16</p>

Question: Do you agree with the choice of language tag for the <dfn> 
element around the very language tag? 

The chose 'zxx' ('No linguistic content') is based on 
<http://people.w3.org/rishida/utils/subtags/index.php?find=linguist&submit=Find>, 
which lists "machine-readable data files consisting of machine 
languages or character codes, programming source code, etc" as example 
of things which could use this tag.

Leif H Silli