From this link
https://www.singaporeair.com/jsp/cms...pointments.jsp
It seems SIA is recruiting cabin crew from all over Asia, but they eventually have to be based in Singapore. With basic salary of S$3500 a month upon graduation, the annual salary with bonuses and other wage supplements common in Singapore, is about S$50k-ish a year, which is usually tax-free because the salary is structured with a low fixed base.
SIA doesn't discriminate based on nationality though, it seems they all earn the same regardless of whether they're from Japan or Indonesia.
SIA is located in a high cost city surrounded by extremely low-cost countries, and has never ever contemplated setting up crew bases elsewhere.
It's an extremely poisonous working environment when one colleague realises the other colleague is getting paid much more for doing the same work on the same plane.