The front-end interviews at Google are about half algorithms and half details about how web front-ends should work, be structured, web technology problems, browser performance, etc. It's an entirely different interview track from General SWE.

How hard is it to ask them to switch me to something more backend? Is it impossible unless I know Another one of their languages?

Tell the recruiter you're a backend developer, and are interested in one of those positions instead.

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.

Good to know!