tlhIngan here is being used to call to someone, speaking to a Klingon. The are a few hints to that. The first hint is the comma separating the word from the sentence. The second hint is the placement of the word at the end of the sentence (where the grammatical subject would go) instead of at the beginning (where the grammatical object would go), next, the use of the jI- prefix tells use that there is no grammatical object - no particular thing being understood, just general understanding. And finally, tlhIngan alone refers to a person and cannot be used to refer to a language.
To say, "I understand Klingon," we would say, tlhIngan Hol vIyaj.
Some of the points that I mentioned above are covered in the Tips & Notes for this Skill, but some will be covered in more detail in future Skills. Since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. If you have questions after reading the Tips & Notes for any Skills, then please return to the forum to ask any questions.