I’m currently working on a multi-lingual offline app, but in my case, the data does not need to be translated – only the interface, which is handled in the modeler.
Our data is kept completely separate for the different language regions. You could do something similar. Link all the entities you show in the app to a parent entity (language), and then set access rules on each entity that limit which data the user can access based on the selected language. Then you would just have to take care of filling in the translated values for the data in an admin panel (online browser page).