A Java developer becomes a better Java developer if they learn alternative concepts that Java doesn't empasize as much. Learning Lisp, Haskell or C is going to be useful, for example.

But a Java developer won't become a better Java developer if they learn C#, which is basically Java with different capitalization conventions.

I guess something like that happens for natural languages. You get those benefits if you actively learn new concepts. The difference of course is that you are unlikely to have done that purely by being bilingual. Many bilingual people just picked up 2+ languages as they were growing up.