The backend interview lets you choose whatever language you want. You'll be asked algorithms, distributed design, code quality, and data-structure questions.

Generally speaking, if you focus too much on a programming language / stack, it comes off as a sign that you're not a good fit, since you emphasize programmer skills, but they're looking for engineers. Except for particular roles, like front-end dev, where they're looking for programmers with experience in the technology stack itself.