Can software copyright applications only use one specific programming language? Or can they be in any language like Java or C++? Thanks!
When you register your work with the Copyright Office, you submit a copy of the work that you want registered.
If you want to register C++ code, you send C++ code. If you want to register Java code, you send Java code. Generally:
If the work is an unpublished or published computer program, the deposit requirement is one visually perceptible copy in source code of the first 25 and last 25 pages of the program. For a program of fewer than 50 pages, the deposit is a copy of the entire program.
More nuanced requirements are in Circular 61, including how to handle redaction of trade secrets.
Regardless of registration, copyright only protects your particular original expression. Copyright only prohibits people from copying that expression. It only prevents somebody from copying it in another language to the extent that the two expressions overlap through the abstraction-filtration-comparison test.