exif.encode_unicode defines the characterset UNICODE user comments are handled. This defaults to ISO-8859-15 which should work for most non Asian countries. The setting can be empty or must be an encoding supported by mbstring. If it is empty the current internal encoding of mbstring is used.
How come ISO-8859-15 works for most non-Asian countries? It's only for West European and does NOT work for Russian, Greek, Albanian, Polish, Czech, and so forth. The default should be 'UTF-8'