Accented and non-Latin characters arrive as question marks or strange symbols when the CSV was saved in an encoding that cannot hold them. Nothing is wrong with your data or with the import: the characters were lost when the file was written, before we ever saw it.
The fix is always the same: save the file again as UTF-8, then import that. Below is how, per program.
Excel
In Excel 2016 or newer: File → Save as, choose CSV UTF-8 (Comma delimited). Plain "CSV (Comma delimited)" is not the same thing and is what causes this. Excel before 2016, and Excel on Mac, cannot write UTF-8 at all, so use one of the other programs below.
Apple Numbers
File → Export to → CSV, open Advanced options and set Text Encoding to Unicode (UTF-8), then export.
Google Sheets
Upload the original .xls or .xlsx to Google Drive, not a CSV that has already lost the characters, open it, then File → Download → Comma-separated values. Google writes UTF-8 by default.
TextEdit and Notepad
TextEdit on Mac: open the file, Format → Make Plain Text, then File → Save and pick Unicode (UTF-8) in the encoding dropdown.
Notepad on Windows: open the file, File → Save as, set Save as type to All Files and Encoding to UTF-8, and give it a .csv name.
Already imported the broken file?
Delete the affected locations and import the corrected file, rather than trying to fix them one by one. If the import came from a connected Google Sheet, just correct the sheet: Google stores text as UTF-8, so the next run brings the right characters through.
Stuck on any of this? Open the chat in the bottom right corner of the app and we will help you set it up.