One of the ways I learn new languages/frameworks is by porting something I've already made before and don't really have to work out how to design it again.

For example, I've ported my Proximity game a bunch of times. Some of these never got completely finished and released:

* Actionscript (Flash, released and playable online)

* Java (J2ME and later Kindle -- the Kindle version looked awesome, too bad Active Content is no longer a thing)

* C++ (Popcap framework)

* C# (XNA, released on Xbox 360 on Xbox Live Indie Games)

* Objective-C (OpenGL ES, released on App Store)

* Python (I just used text graphics)

* Lua (Pico-8, almost done but not quite)

* Physical board game prototype, hoping to find an interested publisher

* C# again (Unity this time, and making it in 3D)