"Korean is not the easiest language."
Translation:한국어는 제일 쉬운 언어가 아닙니다.
The easiest?. The easiest
가장 is more like being among "the easiest".
But, 제일 is being the one language rated as "the most easiest".
So, for this lesson using negative verb, we get the idea that this is not the world's top easiest language. In Korean, by the strictest use of 제일, there can only be one top-ranked of anything.