Answer by Rex for Is it possible to force Excel recognize UTF-8 CSV files automatically?

This is an old question but I've just encountered had a similar problem and the solution may help others:

Had the same issue where writing out CSV text data to a file, then opening the resulting .csv in Excel shifts all the text into a single column. After having a read of the above answers I tried the following, which seems to sort the problem out.

Apply an encoding of UTF-8 when you create your StreamWriter. That's it.

Example:

using (StreamWriter output = new StreamWriter(outputFileName, false, Encoding.UTF8, 2 << 22)) {
   /* ... do stuff .... */
   output.Close();
}