Most languages created for fictional worlds involve simple vocabulary substitutions, such as moodge for man in A Clockwork Orange, or meaningless streams of noise, like the high-pitched jabbering of the Ewoks in Return of the Jedi. Klingon is something altogether different. There is a logic behind it; a linguist doing field research among Klingon speakers would be able to work out the system and describe it as he would an exotic indigenous tongue. This is not surprising, considering that Klingon was created by Marc Okrand, a linguist whose dissertation was a grammar of a now-extinct Native American language.

their foreheads look like that from banging their heads on the wall trying to learn the language.

The Klingon word for hello is "Pogmahone"