It's fine to not know every language. I'm not saying you must know every language. I'm saying that only knowing one and refusing to use another is a problem I've seen from PHP, Java, and C# cultures almost exclusively.

The only exception I'd say that makes sense is people who are using coding for a small part of their overall job. But full time software engineers should have at least a few options in their belt for backend that they understand and can use in different scenarios.