Plain text files do not have an explicit identification of the files' character encoding. If the files are not in the UTF-8 encoding, conversion is needed. For example, you would run the following command to convert a plain text file encoded in Traditional Chinese big5 to UTF-8:
$ iconv -f big5 -t UTF-8 inputfilename > outputfilename
The Text Editor application can read and write character encoding text automatically, or you can specify an encoding explicitly when opening or saving a file. To start the Text Editor, choose Launch→Applications→Accessories→Text Editor.