Earlier quoted context omitted.
One answer to your question is C# / .Net. Combines excellent concurrency primitives, very good performance, and a deep reservoir of packages and libraries. Of course it's almost never worth switching languages for an existing product, you just get left with a mess of half-old and half-new tooling and invariably get left having to support both languages.
Real question: From your specific comment, are there any advantages of C# vs Java here?
In C# Microsoft often provides the most popular tooling for common problems. For instance, in web development, Microsoft publishes ASP.net and Entity Framework. In Java there is Jax-rs (JSR 339) and JPA (JSR 317).
So depending on your preference for a strong centralized environment provided by a single vendor, or a decentralized standards based solution with many vendors, you might lean towards Java or C#.