Excel for Mac can't handle UTF-8 encoded CSV files. Official statement by Microsoft:
Excel for Mac does not currently support UTF-8. If this is a feature you’d like to see in future versions of Office for Mac, be sure to send your feedback by clicking “Help” > “Send Feedback” [...]
Convert the UTF-8 file to UTF-16 Little Endian by running this one-liner in Terminal:
( printf "\xff\xfe" ; iconv -f utf-8 -t utf-16le INPUT_UTF8_FILE.csv ) > OUTPUT_UTF16_FILE.csv
Voila!