Personally I love Ruby the most and use it every day, but which language you should learn next depends on your goals.
Ruby and Python have very similar sweet spots: they are both interpreted, dynamically-typed general-purpose scripting languages. They have similar performance, and similarly mature libraries. The differences between them are essentially philosophical - Python is about doing things the One Right Way, and Ruby is about making developers feel delight by letting them muck about with anything they want.
Go, on the other hand, has a very different sweet spot. It's a compiled, typed language with a focus on server programming and concurrency (which neither Ruby nor Python address very elegantly).
If I were in your shoes, I would learn Go next, because in the long run I would have more options knowing both Python and Go. But if your goal is simply to get more exposure to interesting ideas and approaches, you would be well-served by learning either Ruby or Go.
You're right... It may be better to expands my knowledges... Do you recommend me any good tutorial?
I wish I could be of better help, but not having learned Go myself, I can't recommend anything first-hand.
It looks like the Go Wiki has a section with links to books + videos that might be of some assistance.
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