That's the thing, though -- languages aren't just changes of syntax. Writing in an object-oriented style is very different from writing in a functional programming style, even within the same language.
Alternately, new libraries are worth learning. Learning how to deal with massively parallel code is very different from singlethreaded programs.
Of course, you can write C in any language.
Learning a new syntax is not that challenging, and no language will be the optimal(or even possible) to use for every project.