"Has he arrived?"
I went on a short course in Swahili this spring and handed in an exercise on questions. I put "Je, ..." on every one and got my homework back with all the "Je"s crossed out. The teacher (from Tanzania) said it wasn't wrong, just unnecessary. It would be obvious from the intonation in spoken Swahili. I found his question intonation hard to mimic though, and the other option he gave was to start with "Je, ..." and then speak with normal intonation.
In written text, the question mark at the end makes it obvious it is a question, so I can see why he (and Duolingo) leaves out "Je, ..." at the beginning.
I can imagine "Je, ..." is also useful for a cautious approach to a coming question, like when you say in English, "Tell me, do you like this course?"