The Sheliak language is an exceedingly complex language spoken by members of the Sheliak species. By the 24th century many attempts had been made by the Federation, among others, to comprehend the language but to no avail. By comparison the Sheliak had at this time mastered several Federation languages.
The comparative complexity of the Sheliak language made negotiations over the Treaty of Armens an arduous task. In order to accommodate the Sheliak, who considered the Federation language an irrational one, the treaty was 500,000 words long. (TNG: "The Ensigns of Command")