A FE Eng. is still a software engineer, which means you ought to be able to pick up any backend language, be it Python, Rails, PHP, or a Java/C++ service. A lot of the time you'd be tasked with bugs/features that require a lot of FE work, and some backend work as well; it's so much more efficient for you to just write the backend handlers, esp. when it's usually a stone's throw away from an ORM layer or a simple SQL call.
I'd also consider knowing something about UI/UX to be a nice bonus. In reality, your UI designer isn't going to provide mocks for every page and every interaction; you're going to have to fill in the gaps, and probably do so without a huge dropoff in quality. At the same time, I usually find myself bringing another perspective from the UI side when talking w/ the designer, and since most places only have one UI guy it's valuable for them to get feedback, even if it's from someone not at the same level.