What programming language and OS would you choose to build a fintech app?

In particular, what's important for this discussion is your personal opinion on the matter. Would you pick C#/C++/Python/Java with Linux (via Mono) or Windows? Or would it be preferable to choose Apple's iOS on a par with Swift and Objective-C? What about Android? Undoubtedly, fintech product development is a tough undertaking, so it's difficult to fit everything in one discussion, but any help is enormously cherished!