(Could be from the French "droit de parole," which could mean "[no] right/authority to speak [for/over] Area C." – Papa Poule 2 hours ago)
Thinking more about this I went from "right/authority to speak for/over" to "power of attorney for/over," which brought me back to another French word, "mandat," which led me to "mandate:" "[no] mandate over Area C," from/to which I think one could get "right of say" pretty easily, especially if the document itself had been transcribed/translated many times between several languages before even getting to the final translator.
ADDED Nov. 30
The concept of “mandate/mandate over” as suggested above comes from the use of the term "mandate" by the League of Nations, especially with regard to former territories of the German and Ottoman Empires, but also with regard to Palestine.
Therefore, if the document that you are examining involves these former League of Nations Mandates, especially if originally drafted between 1919 and the establishment of the United Nations in 1945, then “mandate/mandate over” would be historically accurate and appropriate.
Even if written post WWII about these formerly mandated territories (or, in my opinion, even about any other territories written at any time), “mandate/mandate over” would still be an acceptable translation/substitute for “right of say” (and vice versa).
However, based primarily on this site concerning “right of say,” I’d suggest that “control over;” (pouvoir=)"power over;” or (autorité=)"authority over" would be the three best and most clearly understood choices to replace "right of say over" if former League of Nations mandated territories are not involved (I'd pick these three over all the others in the list of "Other Translations," because 1) "control" is listed as the "Related Translation" for both the "autorité/authority" entry and the "pouvoir/power" entry, 2) "power" is the direct translation of "pouvoir," and 3) "authority is the direct translation of "autorité").
(first entry for “right of say” from the above site [second alphabetically after autorité/authority])
RELATED TRANSLATIONS=control; right of say
OTHER TRANSLATIONS=authorities; authority; capacity; command; competence; dominion;
establishment; force; MANDATE; mastery; power; qualification; rule;