-
SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
I am not an expert at all in this area so there may be way better ways to do this. But I really like this library: https://react.i18next.com/ You use a hook wherever you want to translate. The translation can come from a local or remote JSON file. As far as the backend - I would go the route of sending a specific key, like you suggested, or even just a number for each error that you then map to the i18next translation
Try using this library https://github.com/stefalda/ReactNativeLocalization