There has been many good points made in this thread, so I will just add what has not been said already.
While knowledge of the language that you are working with is important, of vital importance is being a developer. Anyone can pick up a book and write working code, but that does not make him/her a developer. You must have (at least) basic knowledge of algorithms, design patterns, data structures, OOP (or whatever paradigm you'll be using), etc. If you have the basics down, you can create theoretical solutions without focusing on the language it will be implemented in. Please, for the love of R2-D2, don't learn a language's syntax and call yourself a developer; we have enough of those. As it is often said that programming languages are tools; use the right one for the job. Good craftsmen are good at their jobs despite their tools, not because of it.
On a different point which might have an influence on your decision: the language you choose to specialise in will likely influence the kind of work that you end up doing. I have observed some trends with regards to Java/C# (and similar) jobs when compared to Python/Ruby (and similar) jobs. Java/C# jobs tend to be at larger companies where "enterprisey" software is developed and mostly in Gauteng. Python/Ruby jobs tend to be at smaller, start-up kind of companies and more common in the Cape region. Now, before I start a flame war for my generalisations, please understand it as exactly that: generalisations. I have, myself, worked at a larger company in Pretoria where I worked on a financial system, developed in Python, so I know that there are exceptions. It is not important to everyone, but the business domain in which I work (which dictates the kind of problems that you will have to solve) is a very important factor to me: I would rather work at a smaller company, with a slightly smaller salary, which operates in an interesting domain, than at a larger company, for a bigger salary, working on some kind of financial system. Again, these correlations are not universal.
In the end, the answer to your question depends heavily on what (more precisely) you want to achieve.