Thanks, 
It seems to work. I've made some tests (quickly beacause I'm going to University right now). It dosen't work with UTF8 ... But it works with "default": 
            FTPConnection connec_ftp = new FTPConnection();
            connec_ftp.CommandEncoding = Encoding.Default;
            connec_ftp.UserName = "projetrt2";
            connec_ftp.Password = "xxx";
            connec_ftp.ServerAddress = "ftpperso.free.fr";
            connec_ftp.ServerPort = 21;
            connec_ftp.AutoLogin = false;
            connec_ftp.TransferType = FTPTransferType.BINARY;
            //connec_ftp.ParsingCulture = new System.Globalization.CultureInfo("fr-FR");
            connec_ftp.Connect();
            connec_ftp.Login();
            connec_ftp.CreateDirectory("