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

I am generating csv files from a simple C# application and had the same problem. My solution was to ensure the file is written with UTF8 encoding, like so:

// Use UTF8 encoding so that Excel is ok with accents and such.
using (StreamWriter writer = new StreamWriter(path, false, Encoding.UTF8))
{
    SaveCSV(writer);
}

I originally had the following code, with which accents look fine in Notepad++ but were getting mangled in Excel:

using (StreamWriter writer = new StreamWriter(path))
{
    SaveCSV(writer);
}

Your mileage may vary - I'm using .NET 4 and Excel from Office 365.