Narrowing topics with branches
One thing that is lacking, in virtually all software or classroom environments, are expanded depths into certain topics. They usually leave you out to fend for yourself after a few words under that broad topic.
It would be cool to see narrower branches added that deviate away from the general progression.
There is a tab on animals, which generally teaches you some common words like dog or cat, but what about branching it out and providing different ways to go with that introduction? You could next go to marine life, or jungle life, or domestic animals. after you reach marine life, you could narrow further into say dolphins. Learning words like foraging and echolocation. Part of learning a language is having something to talk about. You may even learn something you didn't even know in your native language.
In summary a top down branching approach on subjects to provide your alternate language the vocabulary to express yourself would create a ton of content, and ways individuals can specialize themselves for the community.
Great idea, especially the idea of learning interesting facts to motivate you. I would probably say you should start off with only a few subtopics for each skill (once you start learning words like echolocation you'll have learnt pretty much all of the major things about German)