A default file encoding for every file in Netbeans is defined at project level -- see
Project Properties dialog,
Sources category. But there's nothing similar (possible) for particular files without a project. If you're using Netbeans just as an editor then all files opened from outside any project will be scrambled, because Netbeans is not using UTF-8 encoding by default on all files.
Solution is to open
netbeans_default_options= and add
-J-Dfile.encoding=UTF8 to the end of this parameter. It means of course that all files without project will be opened in UTF-8.