If the data you want to import into Yapp includes accented or non-Roman characters, you may have difficulty exporting a CSV file from Excel with those characters encoded correctly. Excel's CSV export does not always handle this correctly. Here are a couple workarounds for this issue:
The first option is to import your Excel file into Google Sheets. Once you have it in Google Sheets, you can export a CSV file with the accented characters preserved by going to File -> Download as -> Comma separated values.
Export as Unicode Text
This option can be done just with Excel, but the process is a bit more complicated. Instead of exporting as CSV, you can go to "Save As". In that screen, select "Unicode Text" as the File Type. You will then have to open up the saved file and replace all the tab characters with commas using Fine & Replace. I would recommend downloading a text editor like Notepad++ for this. Finally, you will want to change the file extension for the file from .txt to .csv.
In each case, you should end up with a CSV file with all of the characters preserved. Happy importing!