Language should be a means to an end. Pick a project you want to work on, then figure out what you will need to know to get the job done. It's very difficult to get any language-learning to stick without some practical application.
And "knowing a language" is not particularly difficult or useful. Knowing how to use most of the important libraries and platforms associated with a language is usually much more time-intensive and useful.
As far as picking a project if you can't think of one, maybe try for something on a mobile platform -- Android / iOS / Windows Phone. These are generally useful for learning not only a language, but a complete set of tools to take an idea from concept to published.
