I can only speak for Australia, but n-gons are not mentioned directly, there are however maths questions that will mention a polygon with n sides.
Technically, most countries use "n" for an "undefined amount".
So that would explain the definition. 
n-gon: poly-gon, penta-gon, hexa-gon, hepta-gon, octo-gon etc.
So technically we all are using it, just that the general categorization, "ngon" is barely used.