unicode_encoding
The default value is what python's sys.getfilesystemencoding() returns, which on Windows is 'mbcs', on Mac OSX is 'utf-8', and on Unix depends on your locale settings.
If you need a different encoding, this can be changed in your build slave's buildbot.tac file by adding a unicode_encoding argument to BuildSlave:
s = BuildSlave(buildmaster_host, port, slavename, passwd, basedir, keepalive, usepty, umask=umask, maxdelay=maxdelay, unicode_encoding='utf-8')