You as bad question- you get wrong answers (Score 1)

"I've been cooking pastry- now I would like to do main dishes- what ingredients should I use?"

What exactly do you mean by "developing for Linux"? Writing drivers? Expanding kernel? Writing desktop apps? Web sites? Command line tools? Distributed programming on Linux cluster? Other, whatever that might be?
Until you answer this question you will get answers all over the map.
Desktop applications- you'll likely use C++ (with some C where APIs isn't properly wrapped by a miriad of OO wrappers for Gnome, KDE or cross-toolkit), however there are great apps in Python and other scripting languages. For lighter weight web site (not necessarily in size, but in functionality) you'll likely use PHP or Ruby. For something more substantive on the Web you'll use Java (and don't be afraid, performance-wise it got quite decent, probably at least ~80% of good C++ app speed).
Ultimately, your choice is limited if you are planning to work in the guts of the system (C with some elements of C++) but beyond that you can use whatever, so you better rephrase the question.