My guess would be it's just because Duolingo doesn't pay attention to punctuation and the way this is worded, it could be a statement, whereas the correct answer is a question. But that's just from my knowledge of Duolingo, I don't know if there's a reason due to the language...
Well, most likely they're stressing the standard word order of questions here. Also, as far as my experience goes, Duolingo doesn't really take punctuation into account when assessing our translations, hence failing to capture the rising tone of your intended question. That's my guess :)
I am having this same problem. To me it sounds like it could go two ways; eng-yask, with a long A and silent L, OR eng-yalsk, with long A and pronounced L. Have you figured it out since you posted this? EDIT: Or even en-yask, with a silent G and long A. The voice is throwing me off, I have no idea what I am saying. PLEASE SEND HELP.
"Eng-" is almost like ing. It's somewhere in between an eh and an ih sound. "-gelsk" is how you would pronounce it in English. They really do say all the letters. The "l" is more in the back of the throat rather than with your tongue. The emphasis is on the first syllable, "Eng-"
I think "Do you speak English?" would be "Snakker du engelsk?", which sounds just a normal inquiry, while the question with "not/ikke" are usually uttered with your eyebrows raised. There is a difference of tone between them I'm afraid, and producing the latter surely needs more context. Hope it'll be of some help :)