add <langUsage> to headers of metadata files

[iljackb]

Though is some descriptions of session you include <lang>Mixtepec-Mixtec</lang> you never actually declare what languages are used in the header, which is a basic feature of both OLAC and IMDI.

[https://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-langUsage.html]

Add the following using XSLT script:

      <profileDesc>
         <langUsage>
            <language ident="mix">Mixtepec-Mixtec</language>
         </langUsage>
      </profileDesc>