There's no way to set it at the moment, as current versions of FTPSync are not fully unicode compliant. See: http://cyberkiko.com/Docs/FTPSync29/KnownIssues.htm
Unicode support is planned, but not before FTPSync 3.0, as this requires a number of changes in the core.
A workaround for some cases is described in blog post:
I have a tool to examine a log file to determine if files were transferred. You can download it from:
Unpack into FTPSync directory, and add to your FTPSync .INI file the following two lines:
AfterTransfer=fsmail.exe mail.yourdomain.com email@example.com "%LOGFILE%"
(replace mail.yourdomain.com with your SMTP server and firstname.lastname@example.org with your email)
This would require Unicode support, which FTPSync doesn't have at the moment. It is planned, but not before FTPSync 3.0, as this requires a number of changes in the core.