The Klingon language is from Star Trek. The details of the language were first put forth in the book The Klingon Dictionary by Marc Okrand, a linguist who was hired by Paramount Pictures to develop the language for the feature films. While not a complete language, Klingon is spoken by some dedicated Star Trek fans and to be able to converse in Klingon may be seen as the mark of a true geek.
In episode 0305, Penny says she told the friend with whom she had hooked up Howard that he spoke five languages, to which Howard replies "Six, if you count Klingon!". Leonard remarks that "Girls don't count Klingon," and Penny agrees.
Later in episode 0305, Sheldon taunts Wil Wheaton during a gaming session at the comic book store by snarling the proverb invented by Marie Joseph Eugène Sue "Revenge is a dish best served cold!"  (commonly considered to be a Klingon proverb) in the Klingon language (bortaS bIr jablu'DI' reH QaQqu' nay' ). Wheaton stares, then asks Stuart, "Did that guy just say 'Revenge is a dish best served cold' in Klingon?" (0305)