In field settings (at "Group help"), dz is listed as "Bhutani".
According to Wikipedia:
"Bhutani" is not another name for Dzongkha, but the name of a Balochi language. The two are sometimes confused, even in some published ISO 639 codelists.
Dzongkha is listed at localize.drupal.org.
Has this to be fixed in Core or in Language field?