I don’t understand why you shouldn’t be able to do it. The last resort would always be to install a Virtual Machine, distrobox or whatever. Not sure about Japanese input. Thought it would work as they stated they support it.
@ReakDuck
Japanese input is doable without any system changes. Mostly adding language as well. Some software does need you to add language packs to system directories - that’s one example where an overlay fs is useful.
Mostly it’s good when you need to add specific packages (I ran into xmodmap not being available for instance). Or if you need something such as a custom keyboard layout (technically possible in your home though).