I am using Win7+xampp, i'm not sure if some folders are/aren't writable, i am an administrator so i suppose they are, on Linux I would know to tell you if they are writable.
Anyway, I've copied the translation folder in ./oc-includes/osclass folder. Translation folder by default existed in the ./oc-includes/ and not in the osclass folder, and it was the problem, now it works fine. I supposed there was similar problem, so yesterday I've copied translation folder to the theme folder and some other folder in the installation, but not in the correct folder.