Автор: http://www.swissdelphicenter.ch

procedure SetDecimalSeparator(Ch: Char);
 var
   DefLCID: LCID;
   Buffer: LPSTR;
 begin
   Application.UpdateFormatSettings := True;
   StrPCopy(Buffer, Ch);
   DefLCID := GetThreadLocale;
   if SetLocaleInfo(DefLCID, LOCALE_SDecimal, Buffer) then
     DecimalSeparator := StrPas(Buffer)[1];
   Application.UpdateFormatSettings := False;
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   SetDecimalSeparator('.');
 end;