I have the EXACT same problem with no idea how to conver the objects (specifically the langage.json files) to local ones. Asked in the discord development chat and so far no replies their either. If someone does solve this please do reply with instructions.
The following piece of that particular file is important in this case:
Fire up the app in your browser and load the inspector tools, try Google Chrome for instance. Take a look at the Network pane and find the downloaded files from the wvservices.com domain. I’ve found the following files and downloaded them locally, made a repo of it and published it at GitHub so I can include my own ‘data-service’ provider, like wvservices: