The FTP protocol does not support Unicode characters per say. So, a FTP server may or may not support Unicode characters, and if it does, it uses a specific encoding type (eg. UTF7 or UTF8, etc.)
If your FTP server only supports ASCII characters in file names, Arabic characters will never show up properly.
By default, Xceed FTP for .NET encodes commands sent to the server and the replies in Unicode UTF-8, which covers ASCII. If you find that your server does indeed support more than ASCII, but its a different encoding than UTF-8, you can use the FTPClient.Encoding to select any encoding you want.