Java and Microsoft .NET are two leading technologies intended for the development of desktop and server-side applications. Both platforms enable the use of high-level programming languages to build large-scale applications. While both Java and .NET are often referred to as frameworks, only .NET is actually a framework in the strictest meaning of this word. In fact, the .NET platform uses ASP.NET as a web application framework to allow developers to create web apps.

As mentioned by other users you can use any language of your choice.
But keep in mind that ERPs are very complex system which you cannot build over one night. It takes long time. So it will be beneficial if you can leverage the built in features of some framework. Becuse the framework take care of most of the things. It separates the user, business logic and database. From that perspective Java (J2EE) is a good choice. I use Oracle ADF to build ERP along with PL SQL and JavaScript.