Question: I was reading about Go, and I can see that it's very good and can be a language used by many developers in some months, but I want to decide a simple thing: **Learn Go or improve my Python or Ruby knowledge?**

Years developing with Python: 1  

Years developing with Ruby: 0.3

Answer: 
How long have you been working with Python?

If it were me, I'd do my best to maybe get a working knowledge of Go (basic syntax, some familiarity with unique language features), and continue with Python as I normally would.

Eventually you might come up with a small project that Go seems suited for (or you can come up with one now!) and really dive into the language that way.

There's no reason to limit yourself to just one. :)
It's not easy to answer this without knowing how good your Python is, or what you do, or where you'd use Go. If you're looking for employment, I suspect Python is the way to go. I'd be surprised to find anyone is using Go for major projects at the moment (outside Google).

Note also that Go is by no means finalised. See [here](http://go.googlecode.com/hg/doc/devel/roadmap.html) for the Go roadmap, and note the potential changes in the future. So you may be trying to hit a moving target currently.