Patrice Vermette was the production designer tasked with the creation of the language.
Of course, if the language was simple, anybody could do it.
The characters aren’t only clueless about what the aliens are saying, they have no idea how to even distinguish words as we know them from the logograms that the aliens create as a means of communication.
In this clip provided exclusivly to CinemaBlend from Paramount, that will be part of the extras included in the movie’s Blu-Ray release, director Denis Villeneuve says the key was to try and create something that wasn’t simply unknown, but was so different any known language that it would be utterly incomprehensible to start with.