The value offered by a language stack like .NET or the JDK is not in the language, it is in the standard libraries which typically run to thousands of classes, and in the non-standard-but-everybody-uses-them libraries like Apache Common or Google Guava. At a minimum you will need to become familiar with those just for basic back end development. If you need to do high performance or throughput, you need to become familiar with the performance characteristics of the standard collections and associated specialized collections (Trove, Guava), you will also need to understand tradeoffs of that technology stack's implementation of low level, mid level and high level synchronization and threading concepts.
If you need to do UI development you need to add a whole layer of higher level libraries (WPF or Swing), understanding their specific implementation of somewhat abstract concepts (MVVM, MVC). If you need to do Web development, you need to understand additional web frameworks (ASP.NET, .NET MVC or JSF, Spring), associated persistence layers (.NET Entity Framework, Hibernate/JPA), front end languages/scripting (JavaScript, CSS, HTML) and multiple data binding approaches, object lifetime (request vs view vs session vs application), authorization and authentication libraries, etc. etc.
I can pick up pretty much any new language over a weekend (except for C++ 1y, that monster takes months), but having more than entry level proficiency with the full technology stack takes months to years.