UN : Umbrella Nutcracker

EU : Egg University

NATO : New Astronautical Telemarketing Orange

ASEAN : Arsonist Seagull English Apple Natives

SAARC : Surfing Ability Award Rooster Crane

BRICS : Brick Raining Icy Coffee Sea

ISRO : Iguana Saw Rat Ostrich

WHO : Watermelon Hugging Ovary

These are the ones I can remember for now-