WebExtensions provide a native messaging API, i.e. a clean way to dialog with an application running on the computer. So you can have your add-on providing the user interface of your translation app, and whenever you need to do something that cannot be done using the regular WebExtensions add-ons API, like say creating a ZIP file, the add-on uses the services provided by the native app.
An example of this is the Video DownloadHelper addon. The latest version uses a companion app to provide services missing from Firefox Quantum, like a file manipulation/writing API, or unrestricted downloads. The companion app that works with DownloadHelper is available in open-source, based on node.js, and provides the full build procedure to generate an .exe installer for Windows, .pkg and .dmg packaging for Mac, .deb and .targz for Linux.