Please read the rest 
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   o  Each encompassed language's subtag SHOULD be used as the primary
      language subtag.  For example, a document in Mandarin Chinese
      would be tagged "cmn" (the subtag for Mandarin Chinese) in
      preference to "zh" (Chinese).

   o  If compatibility is desired or needed, the encompassed subtag MAY
      be used as an extended language subtag.  For example, a document
      in Mandarin Chinese could be tagged "zh-cmn" instead of either
      "cmn" or "zh".
"
"


   As noted above, applications can choose to use the macrolanguage
   subtag to form the tag instead of using the more specific encompassed
   language subtag.  For example, an application with large quantities
   of data already using tags with the 'zh' (Chinese) subtag might
   continue to use this more general subtag even for new data, even
   though the content could be more precisely tagged with 'cmn'
   (Mandarin), 'yue' (Cantonese), 'wuu' (Wu), and so on.  Similarly, an
   application already using tags that start with the 'ar' (Arabic)
   subtag might continue to use this more general subtag even for new
   data, which could be more precisely tagged with 'arb' (Standard
   Arabic).
"