What is your issue? That output looks perfect all accents are OK. What are your expectations regarding the output?
Please explain?
[Added example]
public:
String* filename = "your file name here";
FileStream* fs = new FileStream(fileName, FileMode::CreateNew, FileAccess::Write, FileShare::None);
StreamWriter* swFromFileStreamASCIIEnc = new System::IO::StreamWriter(fs, System::Text::Encoding::ASCII);
swFromFileStreamDefaultEnc->Write("éé");
swFromFileStreamDefaultEnc->Flush();
swFromFileStreamDefaultEnc->Close();