I'm not saying if a developer knows many languages they're a good developer, I'm saying if they're a good developer they know many languages. I should clarify "good" as - experienced, someone who experiments, isn't myopic or fanatical about a particular way of doing things. Someone with great potential may only know one language, but to me that either means they're very new to programming, or they're trying to solve every problem with a hammer.
Kotlin does happen to be a very versatile language. It's great for a lot of different uses, but even with a multi-purpose tool like Kotlin, any real world problem will expose you to a lot of different languages. So IMHO a "good" developer will own the problem they're trying to solve and learn, and a "bad" developer will say "I'm not a X developer, I'll leave this for someone else to solve."
As it relates to the interview process, I find it particularly pedantic when interviews focus on language syntax. It's not an interesting problem, and the whiteboard isn't the same as the development environment. Interviews should focus on how does the candidate solve problems, what is their thinking process? And not, have they memorized the exact syntax for a language-specific feature.