This is an issue with Putty, not Bybou. You need to configure it to handle UTF8.
Putty -> config -> window -> translation -> choose utf in "Recieved data in ..."
See also:
- https://askubuntu.com/questions/16912/ncurses-prompts-look-broken-in-putty-from-windows
- How can I use UTF-8 on Linux from Windows 7 via PuTTY?
- https://stackoverflow.com/questions/859565/correct-putty-configuration-for-ubuntu-9-04
Full disclosure: I am the author and maintainer of Byobu.