Простой пример чтения и записи в файл UTF-8

procedure WriteU8(const FileName: TFileName; Lines: TStrings);
var Writer: TStreamWriter;
    j: UInt32;
begin
    Writer := TStreamWriter.Create(FileName, False, TEncoding.UTF8); // Encoding
    try
        Writer.NewLine := #10; // #13#10-Win, #13-Mac, #10-Nix
        for j := 0 to Lines.Count-1 do
            Writer.WriteLine(Lines.Strings[j]);
    finally
        FreeAndNil(Writer);
    end;
end;

procedure ReadU8(const FileName: TFileName; OutputTo: TStrings);
var Reader: TStreamReader;
begin
    Reader := TStreamReader.Create(FileName, TEncoding.UTF8, False);
    try
        while not Reader.EndOfStream do
            OutputTo.Add( Reader.ReadLine );
    finally
        FreeAndNil(Reader);
    end;
end;