Here is a first patch to add a method setcharset() to the window class.

Using my patch, you can fix your example by adding the line:

   screen.setcharset(<your charset>)

before addstr().

It's an initial hack to fix the issue. Next steps are:
 - use something better than utf8 as the default charset, maybe
locale.getpreferredencoding()
 - copy the charset on new window creation?