Documentation led me to the wrong track, trying to set the charset and form-charset post fields to UTF-8, but this does not work. You always get a windows-1252 encoded notification.
When you search for the solution on Google, you mostly get complaints from developers who struggle with it and pretend there are bugs.
Finally, the solution is obvious (as always). Log to the Paypal web site and go to your profile.
Click the Language Encoding link.
Click the More Options button.
Select UTF-8 in the Encoding drop down list. That’s all!