Sorry, I cannot reproduce this: The file (containing äöü) created by the following program can be read sucessfully by my Excel2007. Similarly, the ods version of the file can be opened by LibreOffice. Please run the program below and try to open the created xlsx files with Excel. Does it work? If not tell us more on your system: Excel version, which language? Operating system?
Could you post a file which shows the issue, as rvk proposed above?
To the experts: My file contains the umlauts directly, not by their HTML codes ("ä", etc.). Is this an issue for xml? If yes, is there a function somewhere in FPC/Lazarus which does the conversion of all non-standard characters to HTML codes?
program myooxmlwrite;
{$mode delphi}{$H+}
uses
Classes, SysUtils, fpspreadsheet, fpsallformats, fpsutils;
var
MyWorkbook: TsWorkbook;
MyWorksheet: TsWorksheet;
MyDir: string;
begin
// Create the spreadsheet
MyWorkbook := TsWorkbook.Create;
MyWorksheet := MyWorkbook.AddWorksheet('My Worksheet');
MyWorksheet.WriteUTF8Text(5, 5, 'äöü');
// Save the spreadsheet to a file
MyDir := ExtractFilePath(ParamStr(0));
MyWorkbook.WriteToFile(MyDir + 'test.xlsx', sfOOXML);
MyWorkbook.WriteToFile(MyDir + 'test.ods', sfOpenDocument);
MyWorkbook.Free;
end.