Sharepoint development in what language?

C#.

You CAN use any .net language that you want of course, but the vast vast majority of SharePoint books, documentation and Code Samples are in C#. The official Microsoft SharePoint SDK Samples is only in C#.

You really want to use C# if you have to develop for SharePoint, especially if you're just starting.

You can use any language supported by the .NET Framework, Visual Basic and C# included.

You'll never know for sure. It could be any language that can compile into a .Net assembly. I think the advice here is that it is most likely C# but it would be hard for someone to say definitely that every code behind in the product was written with C#.

In short, write with what you know and like. If you are just learning and don't have a preference, stick with C#.