In the long system how is a billionaire called? A milliardaire? That doesn't sound right.
https://fr.wikipedia.org/wiki/Liste_des_milliardaires_du_mon...
That makes millionaire and billionaire about as unenglish as milliardaire, I think it just doesn't sound weird to you because you see them more often.
Guess what? Some people (70-80% of the world) speak other languages.