Having enjoyed my experiences with Swift, I decided to play with a new programming language each month, so since July 2014 I've had a Programming Language of the Month:
The amount of work I've done in each language has varied hugely. I only did a small amount of work in Prolog to familiarise myself with the basic concepts, but I've written semi-large programs in all of the others.
Learning, or at least experimenting with, a new language is a tremendous adventure for any programmer. Its been a valuable experience picking up ideas in a variety of functional, imperative and declarative programming languages. If you've got the time, I'd give a PLoTM a try.
*As in new to iOS/OS X platforms.