In this specific context I believe that "President of the Notaries' Association" is an appropriate rendering for "Decano del (Ilustre) Colegio de Notarios". In this context "colegio" obviously refers to a "professional association" (rather than "college"), and the head of a professional association in English is most often a "president" (rather than a "dean"). For example, the head of the American Bar Association (Colegio de Abogados) is its "president" ("decano" in Spanish). Specifically in relation to notaries, the head of the "Louisiana Notary Association" ("Colegio de Notarios de Louisiana"), who are civil law notaries as in Spain, is its "president". Other professional associations that have "presidents" as their highest-ranking officer are the American Institute of Architects, American Nurses Association, International Air Line Pilots Association, Canadian Association of Journalists, etc. So the highest office in a professional association can appropriately be rendered as "president."

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Note added at 1 day15 hrs (2009-08-14 07:53:19 GMT)
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For info:

Other notaries' associations that have "presidents" as their head officer include:

--International Union of Latin Notaries
--National Notary Association of America
--The Notaries Society (England/Wales)
--Council of the Notariats of the European Union