"if there are Spanish-speaking users"
There are. You can tag any post with "spanish" and it will go into this sub-category of Steem which a Spanish speaking user could visit directly: https://steemit.com/trending/spanish
"If there are incomes how they are generated, what they depend on."
I'm kind of a simpleton, but the way I understand it is authors are paid from newly created coins on the Steem blockchain. So essentially, blogging is the mining process.
"How to make the change from steem to a fiat."
You would just withdraw your Steem from your account here to any exchange website, and exchange it for bitcoin. So it's basically as simple as going from bitcoin to fiat, and just depends on what banking or local trading options you have.