Chaoti, well, I tried activating Pie-register on a completely unadulterated fresh copy of wordpress localized in russian. In a standard wordpress registration page those fields are translated.
But in pie-reg they still are “Username” and “Password”. Do you know how exactly the plugin gets those labels from wordpress? This would help, as I could then seek help on WordPress forums to solve this.
And yes, MaxM, consider looking into registration-form.php for First Name and Last Name. I think, it was there, where I have found the message “this field is required” which appears if you don’t fill out a required field during registration.
I wish, all those labels and messages were in the .po file.