How to translate fields for registration form and user profile

You may translate non-system fields for the registration form and user profile at:

Admin Panel -> User Groups -> [the_user_group_you_need] -> Edit User Profile Fields -> choose the field you need -> change the "Caption" value.

You may translate system fields in /system/lib/users/User/UserDetails.php. For example:

'id'        => 'username',        //the “User name” field

'caption'        => 'User name',        //you need to translate this caption.