Alimango Island is actually a real island in the Philippines. It’s located in Negros Occidental, in the middle of the Visayas. Now whether the island in Fate/Zero is really Alimango island, what we can technically be sure of is that they used a Filipino Island setting for Episode 18.
Alimango is Spanish for crab, but also Filipino for the same word. The whole island with the church, the houses, the roosters . . . it’s definitely a Filipino island. Fate/Zero probably used it because Philippines is one of the only countries in Asia with Christianity as its main religion. Also the whole Kerry thing feels very “Bisaya” (no racism intended).