1. Enough about some DB (pick one and learn it in the raw; design, admin, operations) that you can leverage that when using the various DB modules.
2. java's probably past its prime </trollbait> but so too is Assembler. Nonetheless, a background with (any) assembler -- going all the way back to RT11 or CP/M -- will force you to focus on practices and techniques. Those aren't directly transferable, in most cases, but they can help you with disciplines you'll need in many languages... including, most likely, those that aren't even well-known, widely-used... or even, written, yet.

But really, learning any additional language well is -- IM(not so)HO likely to be beneficial.

Updated: added last para; clarified my intent re databases.