Dear Bruce,
Indeed, the information is difficult to find in the documentation. We use WIPO standard ST36 for PATENTSCOPE (
http://www.wipo.int/export/sites/www/standards/en/pdf/03-36-01.pdf) that says:
51. The lang attribute normally contains a two-letter code based on ISO standards for the language of the content of the element to which it is attached. In cases where the two-letter code is not adequate, offices are encouraged to follow the conventions established by the Internet Engineering Task Force and described in Tags for the Identification of Languages(
http://www.rfc-editor.org/rfc/rfc3066.txt).
The ISO two letter code standard (ISO 639-1) is quite well known (see
http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes).
Best regards,
Christophe