Best way is to always test. Here is a nice PS script to check the encoding. Without doing that - if I remeber correctly it’s ASCII.
Sidenote - it doesn’t save the excel, but a datatable. Saving as excel is different (uses Interop instead).
The WriteCSV activity does not support supplying an encoding parameter. You will need to roll your own activity/class/extension method (or make a workflow for it, but the compiled code will be faster and more memory efficient).
Here on SO are some examples (in C#), as well as here on MSDN is the WriteAllText overload I’d use (notice the Encoding parameter).