Is there a possibility to develop a chatbot with Watson Assistant in other formats than the native JSON (or with UI)? For example XML format or plaintext that would be then converted into the native WA JSON.
Yes, there is an open source project called Watson Assistant Workbench (WAW) that allows developer to use a few additional formats that can be used to develop a chatbot in Watson Assistant. Among these XML format is supported as well as CSV format (for intents and entities) and Microsoft XLS format (for simple question-answer dialog flows).
The project can be downloaded here: https://github.com/IBM/watson-assistant-workbench . One of the big advantages of this is that as all the files of the chatbot are text files, this can be then held into the github so the developer can use github versioning to track the changes.
The workflow is that the user defines his/her chatbot using WAW and then converts the definition into WA JSON format that can be than imported (either manually or even automatically) into WA.