I agree with you ... we basically always use the simple present to talk about future actions within a "when" or an "if" clause ... the "will" in the main clause is enough. This might be one of the cases of the creators' English being a bit odd, or they might have decided it was good to give us a hint to use the future in Swahili rather than the present, as we would in English. In any case, it should be reported so the team can make a decision when they eventually get around to it.
We'll go unfortunately is a function of the program that handles contractions. While contractions are normally ubiquitous in English there are places where we just don't use them - and Duo allows them everywhere and suggests them often as the correct answer. I'm not sure how they fix it except by requiring teams to put all contracted forms in separately as correct answers.