"The Klingon understands Klingon."
Translation:tlhIngan Hol yaj tlhIngan.
It means "language". If you click on (or on a computer, hover over) the word, a little hint box will pop up. The hint look up tool doesn't work for words that begin with an apostrophe, but it should work for Hol.
The word tlhIngan refers to the beings only. It can be used in front of other nouns to talk about the the things belong to or connected with the Klingons. In English we can use the name of a people to talk about their language: Chinese (both the people and the language), Spanish (both the people and the language), English (both the people and the language). In Klingon you can't do that. tlhIngan only refers to the people. So you have to say tlhIngan Hol to talk about the language.