[eonil]

Obviously C/C++. The most insane choice but also the only sane choice. C/C++ is supported by literally EVERY platform. Whatever platform you try, it will support C/C++, so that's most useful. Even you can't find usefulness from it, it will make you to feel any other language just easy.

[pairing]

I agree with @eonil. I'd recommend learning C. As someone coming from a Ruby background, the ability to understand the ruby internals could be very beneficial. I've started on this path myself. I bought an Arduino to make it a fun electronics / C side project.