This has come up before on Proz. The head of a professional organization is generally called a "President". The "Decanato" could thus be rendered appropriately as "Office of the President"

NOMBRE, firma delegada del Decano = NAME, signing on behalf of the President

Here are US, UK, Australian and Canadian websites in which indicate that the head of an association of notaries (Colegio Notarial) is called a "president" (rather than a "dean") taken from a previous Kudoz question:

Marc L. Aronson, president of the United States Notary Association (USNA) and the Pennsylvania Association of Notaries (PAN) has been ...
blog.enotary.org/2009/07/usna-president-named-to-pria-board/

The National Notary Association, the leading organization...said Milt Valera, National Notary Association President. "Home mortgages. ...
www.encyclopedia.com/doc/1G1-141482011.html

President, Louisiana Notary Association, Baton Rouge, LA. Meeting Parliamentarian - February 2001 - Annual Meeting ...
www.alanjennings.com/cajresume.htm

Judy Chapman, President of North. Carolina Notary Association, was nominated as “Notary of the Year. ...
www.ncnotary.org/.../nc_notary-vol_1_or_4_2003-completed.pd...

He is President of The Notaries Society of England & Wales necessitating travel far and wide giving papers on modern notarial practice. ...
www.roebuckslaw.co.uk/people.html

The President of the Society of Notaries in Victoria, Mr Michael Bula, said after the visit that “the global, legal, political and economic ...
www.notaries.org.au/press.html

I had initiated the association with the Quebec Notaries back in. 1990, when I was a Vice President of our Notary Society. I also had the pleasure of heading two delegations of ... 1 of only 2 in Canada . . . and very rare. ...
www.notaries.bc.ca/.../PDF/WHEREARETHEYNOW-winter2008-46.pd...

http://www.proz.com/kudoz/spanish_to_english/law_general/390...