One thing that is different from docker however, is that we generally want translations/locales in the runtime. But these are pretty large, so we want to handle them in a smart way. xdg-app has a feature that allows splitting a runtime into a base and a set of optional extensions that are mounted in the sandbox only when they are installed. I use this in the gnome runtime to split out each locale. Unfortunately Fedora uses a single large locale-archive file for its locales, so its hard to do that. This needs some research to figure out how to do properly.
Fedora Internationalization team working with Glibc to split locale-archive. This is in plan as a change proposal for F23. Hopefully we will see good progress soon. Cheers, Pravin Satpute