YIGO. YOÑA. Why the Y?
1. The Y has been used to reflect the Chamorro sound DZ for a long time.
Blessed Diego Luis de Savitores, even before he set foot on Guam to begin the first mission, was already using the Y when he spelled Chamorro words. He had learned basic spoken Chamorro before he landed from a Filipino survivor of a ship wreck who spent 17 years in the Marianas.
The use of the Y in Chamorro village names is seen in the earliest Jesuit maps of the Marianas, as well.
2. The DZ sound is not found in Spanish.
When Sanvitores and the Spanish missionaries came to the Marianas, they were faced with a language which included sounds that were not used in Spanish.
The Spanish language does not have the English J sound nor the DZ sound. "Juan" is pronounced "Hwan," not "Jew-an." The Spanish value of the J is seen today in Inarajan and Sinajaña.
3. Some Spaniards, and others in Latin America, pronounce the Y as an English J.
That being said, a good number of Spanish-speakers, whether in Spain or in Latin America, often pronounce the Y as an English J.
Listen to this woman tell us in Spanish that she is from Colombia. In Spanish, "Yo soy de Colombia." The first time she says "yo," it sounds very much like "joe." The second time is not as strong a J and the next two times it's back to sounding more like a J.
4. The Chamorro Y is not said like the J in English. It is pronounced more like DZ.
It is a dead giveaway that someone is new to the island when they pronounce Yigo as GEE-GO, and not DZEE-GO.
But for the Spaniards, the way some of them pronounced the Y (like in the video above) was the closest thing to the DZ sound in Chamorro, and perhaps that is why they used the Y.