All character strings in the kernel structures are always UTF-8 encoded, even if the input XML file was in some other encoding. If your export format requires some other character encoding, you can use the libiconv library to transcode strings into other encodings.
If your exported format includes any messages that can be translated into other languages, please enclose them in a gettext translation macro like this:
fprintf(file, _("Hello world!"));
This way messages in your export module will be included in the Tablix translations. See GNU gettext documentation for more information.