It seems to my case that Excel 2011 for Mac OS is not using Encoding.GetEncoding("10000") as i thought and wasted 2 days with but the same iso as on Microsoft OS. The best proof for this is to make a file in Excel 2011 for MAC with special chars, save it as CSV and then open it in MAC text editor and the chars are scrambled.
For me this approach worked - meaning that csv export on Excel 2011 on MAC OS has special western europeean chars inside:
Encoding isoMacOS = Encoding.GetEncoding("iso-8859-1");
Encoding defaultEncoding = Encoding.Default;
// Convert the string into a byte array.
byte[] defaultEncodingBytes = defaultEncoding.GetBytes(exportText);
// Perform the conversion from one encoding to the other.
byte[] ansiBytes = Encoding.Convert(defaultEncoding, isoMacOS, defaultEncodingBytes);
decodedString = isoMacOS.GetString(ansiBytes);