At the same time, this approach assumes your candidate is currently working on the language you use in the interview. I think it's not rare to have people working on different stacks depending on the projects, and they can be expert in front-end dev for instance, but have been diving into BE APIs for weeks when you interview them. Some can be coding in any language they know in seconds at will, others will need a lot more time to switch context.

It's a valid approach I think, but not without it's pitfalls.