Store dictionary data in application data instead of cache
I'm not sure if there's a technical reason for it, but Nani downloads and stores dictionary data in the app's cache instead of the app's data. As application cache is usually frequently cleared en masse to save space, this means it's much easier to lose and thus redownload the dictionaries than if they were part of the app data. In my phone's settings along (Huawei Mate 10 lite running Android 8.0), Clear Data has a confirmation prompt but not Clear Cache which is done instantly.
This is the storage of Nani with the radicals and JMDict_e dictionary installed: