What do I need to write a small game on Linux?

Some Python recommendations (I seriously think you will not regret it as a language choice ; given your needs):

Pygame would be terrific for writing a small game (especially for 2D). Good (community based) support, documentation, examples, cross-platformness for free...

If you want to go 3D Ogre3D may be worth a look.

Python + PyGame.

Ok. C in general (Visual C#, C++, C, Objective C, and so on) has many pros. It is fast, and I mean fast. However, it takes a while to develop something simple. Java is a cross between C (being hard to learn and taking a while to develop something) and python (lightweight and powerful). Python is what you should learn in my opinion. It is easy to learn. It is powerful. It is lightweight. It is cross-platform compatible. It is flexible. It has so many libraries for it, you shouldn't have trouble finding the right tool for the job. That is my opinion.

This may not be exactly what you were looking for but I would say this is what you need most: "stop planning, start building."

You're trying to compare A to B to C to D, when you apparently don't really know any of them as well as you want.

Start by picking one of them that you want to know (say Ruby, since it's at the start of your list), and start trying to do what you want. You will partially succeed, and partially fail, and both of these outcomes will contribute both the programming knowledge you want, and you'll either finish your game, or gain enough knowledge that you can make an educated choice about moving to another platform.