Ask IH: Tech stack for building custom chatbots

hi there,
I am working on a recruiting-related product where hiring managers or recruiters can add their job postings. A few customers have asked for custom chatbot for their individual chatbots.
What is the most efficient way to build custom chatbots?
For the first iteration, we can add the chatbots for the customers and later provide a dashboard for them to build.
We have a very simple tech stack.
UI: Angular4
Backend: Java8
Database: Postgres
Thanks in advance.
Best,
Himanshu Yadav