They're good to know if you'd like to develop or change a program which is
then compiled to binary (assuming you have the source code, of course).
You've probably heard of C being used for the Linux kernel and C++ for
anything else. C++ works very well with C, but sometimes it requires small
adjustments in the workflow to do the opposite.
Soon.to.be.Free's suggestions are what you should be looking at though.
Anything to do with Debian packaging and package customization would be a
good first step. If you run into a certain language extensively and you don't
understand it, that would be the time to start learning it.
As for creating a hardened version of Debian, you can look for free software
that already exists to help with that, and configure your distro to utilize
that software. It's also good to know the impact each piece of software has
on another if you're concerned about security, as exploits exist to take
advantage of those relationships.
