I had to do something similar and what worked for me was this:

Right click on the file and Open with... Google Chrome. Or if that doesn't work, open with Safari and then from the View menu, choose Text Encoding> and then choose Big5 or whatever it is.

Next, Cmd-A to select all the text.

Now paste that text into a new blank document in TextEdit.

Now Save... and make sure to choose UTF-8.