I'm working on a program that is writing a UTF-8 text file. This process works fine. What doesn't work fine is that TextEdit.app (and others) does not recognise it automatically as a UTF-8 text ...

Notepad started using UTF-8 as the default character encoding – it used ANSI as the default encoding. Let’s assume that you have a text file showing some unusual characters such as “ð???”.

In OpenOffice, change the Character Set setting to "Unicode (UTF-8)" when the Export Text File dialog box appears. Google Sheets saves the characters as UTF-8 automatically.