Update comments about language codes

Update comments about language codes

* RFC 3066 is obsoleted by RFC 4646.
* RFC 4646 is obsoleted by RFC 5646.
* All are also known as BCP 47.
* Language codes must treated as case insensitive.
* There exist conventions for the capitalization of some of the subtags.
* Write a space between BCP and 47 in comments.
* http://www.adobe.com/devnet/xmp/pdfs/XMPSpecificationPart1.pdf is not available.
* https://wwwimages2.adobe.com/content/dam/Adobe/en/devnet/xmp/pdfs/XMP%20SDK%20Release%20cc-2014-12/XMPSpecificationPart1.pdf
  is the new position.

Change-Id: I91e305fd5754cf4dedf8fd7460f7d3375f792c89