There are plenty of Star Trek fans that have made it their mission to learn Klingon. As startrek.com points out, there are meet ups, books and the Klingon institute. But, it is an absolute stroke of genius for Duolingo to make the harshest language in the universe so accessible.
You can learn phrases in Klingon like, “Dlvl’ Hol Dajatlh’a” (Do you speak English?). That’s essential. But you can also learn to make threats, in case the federation needs to take action. Things like, “Hegh tlhIngan” (A Klingon will die). Though, you’d better be ready to fight, because Klingon doesn’t have much to offer by way of apology or backing down.
You can go to duolingo.com to get started. Practice hard, because I can’t imagine it’ll go over well if you mispronounce the wrong phrase. A major intergalactic flare-up is more than feasible. But, nail it and you can declare “Qapla’ batlh je!” (Success and honor!)