Translation:I learn Japanese.
That is true for most writing systems, with a few exceptions. The thing is, they have mostly evolved away from the images they were supposed to represent, and only the sound values remain. That is to a certain true for Japanese kanji as well, but you can still for many of the symbols still recognise the roots.
I can kinda notice it because the "o" at the end of 日本語 sounds longer, but I guess it takes some getting used to it. And I'd say that any language becomes harder to understand when you listen to a natural conversation. Actually, in my experience casually spoken Japanese is way less incomprehensible than casually spoken English for a learner.
Though that, I also think that these sentences are faster than what you would expect for beginner level (although as you can hear them over and over, and have the transcription, I guess that's not so so bad. Actually it's a great training)
習う is receiving instructions to learn, while being taught such as in a classroom. It implies that you are actively being taught. 学ぶ「まなぶ」 is learning. The translation here is somewhat incorrect, and could also be understood as "I study japanese" because of implications, but "study" is not accepted as an alternative to learn here.
"I'm learning Japanese" is accepted as a translation here.
I'm guessing that the verb here shows a greater divergence in use for the present progressive between English and Japanese than for most others. After all, English is unusually flexible with "I'm learning." You can say it basically as long as you've at least started at some juncture in the past. It doesn't have to be what you're doing this moment, and, really, you don't have to have spent any time on it in quite a while.
What does this really mean? "I learn Japanese" doesn't really make sense in English. Grammatically it is fine but it doesn't really communicate any sensible concept without some extra information ("I learn Japanese by watching anime"). Perhaps the Japanese means something more like "I study Japanese"?
(someone) learns / is learning Japanese is the answer Duo's looking for. You have a few possible options for the (someone), since it's not specified, so sometimes Duo mixes it up a bit when it's giving you an example correct answer.
It's important to realise that, because sometimes you make a mistake and the example answer has more than one change - your error is corrected, but something else is different too, and you might think that's also a correction. Sometimes it's just another valid option, like he instead of I in this case
Ideally they wouldn't do that, and they'd keep as close to your answer as possible while only changing the wrong parts, but hey. As you get more familiar with the language you'll learn to see where you messed up and what works!