"I speak Klingon."
Translation:tlhIngan Hol vIjatlh.
I've gotten a lot of sentences throughout lesson one that have said I made a typo when I didn't - everything down to the punctuation was the same. For example, on this sentence, it said that I made a typo on vljatlh. I don't know if that's something that the contributors have any power over or if Duo needs to fix it, and it's still counted them as correct and let me move on - but wanted to let you know.
Unfortunately Duolingo uses a font which makes it difficult to see the difference between an l (lower case L) and an I (upper case i). If you look carefully, you can see that the lower case L has a small curl at the bottom that is missing from the bottom of the capital i. Compare the l where you have typed the sentence to the I in the translation given by the software. I suspect you will find that all of those rejections when you thought you were right, have an l substituted where an I should go.
In English we often use the adjective for a people to also refer to their language. But in Klingon the name of the people is only used for the people themselves and to use it to refer to their language, their planet, their ships, etc. you have to place it before the thing you are labeling tlhIngan Hol ("the language of the Klingon people" or "the Klingon language"), tlhIngan yuQ ("the planet of the Klingon people" or "the Klingon planet"), tlhIngan Duj ("a ship of the Klingon people" or "a Klingon ship"), etc.
"Language". In English we usually just use the adjective for the people or the country as the complete name of the language: "Spanish", "Chinese", "German", "Klingon", etc. In Klingon those words only apply directly to the people and if you want to use them as an adjective, you must include an additional noun to attach them to. tlhIngan is a person. tlhIngan Hol is a language.