Translation:They have a dog, which has four puppies.
Actually, I don't mind the literal translation to which, because it helps me speak Finnish better, and understand Finnish better. Now, if this were a course on English, it might not be right to choose which over who. For me, as a non-English native speaker, using who sounds weird anyway, because we are talking about a dog.
by the same token, you might want to omit articles in English translations because they do not belong in Finnish, or gendered pronouns, or any feature that makes the two languages different. besides, Duolingo penalizes user errors in English translations, so they shouldn't make any themselves.
I get where you are coming from, but that's not what I am saying. I mean that understanding Finnish is the goal. Articles are necessary for understanding. So are many other differences. The basic goal is to learn Finnish, not to have the English translation tuned to perfection. If I as a non-native English speaker say 'which' in stead of 'who' where a native English speaker would say 'who', thats a nice discussion in the English language. Does it improve my understanding of Finnish? I don't think so.
I'm a native Polish speaker and my first learning encounter with Finnish was in a Polish speaking environment. Like Finnish, Polish does not have articles and I can assure you, skipping articles would make it much easier to learn and understand Finnish. Also, please bear in mind my second argument from my previous comment: Duolingo penalizes user errors in English translations. I think the Duolingo team themselves should avoid errors in English for this particular reason.