osx - opening a unicode file in text edit
You can set the default opening encoding to UTF-8 or UTF-16 in TextEdit Preferences/Open and Save.

But your example seems to indicate a non-unicode text, where Latin-1 ä (E4) is being read as if it were in MacRoman encoding, where E4 is allocated to ‰.

Or perhaps it is Unicode UTF-16 being read as if it were MacRoman?