I have my doubts about that claim, but I'll explain those later.
But, according to the Guam News Letter in June of 1915, the first exchange of vows performed in the English language of Guam was between David A. Diaz and Rita Millinchamp Duarte.
The bride was a Chamorro mestiza, the daughter of the Spaniard Pedro Duarte y Andújar and the Chamorro mestiza María Victoria Anderson Millinchamp. María was the daughter of Henry Millinchamp, a man of English and Polynesian ancestry, and his wife Emilia Castro Anderson, a woman of mixed Chamorro and Scots ancestry.
Díaz was not Chamorro but had rather come to the island connected somehow to the U.S. Naval Government. He was a member of the Agaña Lodge of the Elks Club. His mother resided at the time in San Francisco, California. He could have been of Mexican ancestry, but his surname reveals he could have been from any number of Hispanic backgrounds or even Portuguese.
The wedding took place at the Agaña Cathedral on May 22, 1915 with Påle' Román officiating. Påle' Román was one of the few priests on Guam at the time who had a good grasp of English.
REALLY THE FIRST?
My doubts about the claim that this was the "first wedding ceremony" in English here on Guam stems from the fact that other American-Chamorro weddings were celebrated on Guam before 1915. James Underwood, for example, married Ana Martínez about ten years before this, in the early 1900s. It is possible that Underwood, an American, pronounced his vows in Spanish or Chamorro, but it seems just as likely that he said them in English.
There were other American-Chamorro unions before 1915, as well.
Still, the Guam News Letter said what it said and we can't just dismiss the statement entirely. I'll just leave a little room for another possibility.