A Matrix server with a nice web based client, hosted on premise. People are then free to choose the mobile or desktop apps they want.

Is there a specific reason your translations are in json files, and not using a system such as gettext? IMO those .po files are much easier to translate for non-tech people, especially with many third party apps available for editing them. There are libraries available for many languages, so you don’t have to do anything manually in most cases.