So my reasoning is that if you know some Java you can pick up C# easily, if you know some JavaScript you can pick up Python easily etc. If you've stuck to a single language, it's going to take you much longer as you're having to learn new concepts and it's also rare I work on projects where only a single language is being used. Everyone should aim to learn enough languages that they have at least some experience of OOP, functional programming, dynamic types, strong types and SQL in my opinion.