If you're working with Java, you might as well use the Java locale format (en_US).
The BCP 47 documents actually do specify the en-US format, and it's just as common if not more common than Java-style locale names. But in practice you'll see the form with the underbar quite a bit. For example, both Java and most POSIX-type platforms use the underbar for their language/region separator.
So you can't go far wrong with either choice. But given that you're writing in Java and probably targeting a Unix platform, en_US is probably the way to go.