Hello, @yairmakhivskyi 
Unofficial answer. We can wait for the pipefy team staff
The programming language used in the development of Pipefy is mainly JavaScript, both on the client side (frontend) and on the server side (backend). JavaScript is a widely used language for web development, allowing the creation of interactive and dynamic interfaces in the browser (using libraries and frameworks such as Angular, React or Vue) and is also often used in server-side development with technologies such as Node .js.

Hi @yairmakhivskyi ,

You can find the answer to your question at https://stackshare.io/pipefy/pipefy. This website provides a comprehensive list of the tech stack used by Pipefy.

The platform is based on a microservices architecture and uses a variety of technologies. I hope this helps! Let me know if you have any other questions.

@marcosmelo, @cassiano-alves  — thank you so much for the answers, colleagues! Your insights are the goldmine for me.