The final language on my list is a slightly unusual choice. All of the others are fairly flexible general-purpose languages. Prolog is not. While it is Turing-complete, and thus can be used to implement any algorithm, doing so is not always sensible.
Prolog is based on predicate logic, and is very good for building knowledge-based applications. Any system that collects arbitrary relations between objects (or concepts) and performs some inference or reasoning about them is a good fit for Prolog.
The real reason for learning Prolog, however, is that it’s about as unlike C as possible while still having a useful language. Once you can think in Prolog as easily as in an imperative language, you’ll find it very easy to adapt to new programming models.
Like functional languages, Prolog doesn’t allow global variables. In fact, it doesn’t allow assignment in the classical sense. Variables in Prolog can have only one value for their entire lifespan—a characteristic that Erlang inherited. The first Erlang implementation was written in Prolog, so the syntaxes of both are quite similar.