What programming language does the AI assistant use when it builds an app?

Whatever language you want. The AI assistant has its own sandbox, effectively its own computer, so it can install and use the languages and tools you ask for.

If you specify a language or a stack, for example a React app with Node, it uses that. If you do not specify, the default is Next.js and React, and otherwise the choice is up to the AI. If you do not care about the stack, just describe the outcome you want.