Nation by Terry Pratchett

From chapters.ca:
Alone on a desert island — everything and everyone he knows and loves has been washed away in a storm — Mau is the last surviving member of his nation. He’s completely alone — or so he thinks until he finds the ghost girl. She has no toes, wears strange lacy trousers like the grandfather bird, and gives him a stick that can make fire. Daphne, sole survivor of the wreck of the Sweet Judy, almost immediately regrets trying to shoot the native boy. Thank goodness the powder was wet and the gun only produced a spark. She’s certain her father, distant cousin of the Royal family, will come and rescue her but it seems, for now, that all she has for company is the boy and the foul- mouthed ship’s parrot, until other survivors arrive to take refuge on the island. Together, Mau and Daphne discover some remarkable things (including how to milk a pig, and why spitting in beer is a good thing), and start to forge a new nation.