Hello,
Thanks everyone for replies.
Dave, I already choose a preferred domain
Chris, thanks for the useful tip. I think I already got penalized for this (eg I have 3 blog posts all with the keywords "website ranking" in the url, all for different seo strategies, and only one page is ranked for those keywords). I think i only could 301 redirect, but at the moment, I will leave it as it is, and be more careful in the future.
Tuzzel, thanks, your reply really helped also. I have seen in the Google guide you posted that I should use top domains for translations. This is not possible at the moment. Then you suggested to use canonical tags. Does it mean I should 301 redirect all translated pages to the original ones?