Dear Debbie,
No doubt that your open mind will allow you to understand that "The corresponding lists of administrative languages were established jointly with a work of the UNGEGN" does not necessarily mean that "These lists were established by a joint working group with the UNGEGN", but can also mean that "Two groups (UNGEGN and TC 46/WG 2) working simultaneously on the same questions, with somebody being member of both groups, whose conclusions and results were quasi-identical".
I join a note in french about this work.
Gérard Lang