Terrible English. On the other hand, if you translated it in a proper sounding way, ("he is a person of great gentleness", or sth like this) , nobody could ever guess the translation. Duolingo is based on translations , which is nice and useful, but teaching common Swahili expressions that can't be properly translated is a problem
There's plenty of problems in the course, but I'm not convinced this is one of them. If this is in fact a common phrase in Swahili, then we should learn it. So what if it's not common in English? That's even more of a reason for us to learn the phrase because it will catch us off guard. The translation was not hard to figure out.