I have a question about translating existing themes. When i download a free template and translate it e.g. into spanish, am i allowed e.g. to add an additional link to the footer like "translated by xyz.com" and then redistribute it again in Spain as a free spanish template?
If it is in the terms of use that you are free to modify and redistribute it...then yes. If it is not, then no, you cannot. Every theme and theme designer is different, they don't all fall under a general rule. You'll have to read each to find out which allows this. Even if it is not allowed, you could still contact the original designer/distributor and ask for attribution. Many welcome translations and will gladly list you.