Depends where your job focus and interests are. If you're working on webby systems, then figuring out python or ruby might be useful for your next job - a lot of the background knowledge is the same, but you need the language knowledge to be able to apply it. If you're really all about the systems, the a good knowledge of C would help when you need to change the bigger tools you're using for your own needs. Or for large system automation/integration stuff, look at make, no doubt you use it regularly, but I suspect you have little idea of what it does, how it does it, what it can be used for aside from making compiling C programs easier. Or there are plenty of other aspects that come from sysadminning, whether it's db development, db admin, networking, all can grow quite easily out of the same role.
in reply to Learning ***** as a second language