There isn't a way in Jubula to export the data in UTF-8, but you could use another tool to convert the file after the export if something like that is available. I'm not sure if there is a specific reason why we export in UTF-16 - you could put an enhancement in to change to export format if you want to.
Yes, I wondered about UTF-16 as well. Thought it would be UTF-8 per default but it hit me hard when trying to do resource filtering in Maven using that xml file. Maybe it's possible to do an enhancement to allow choosing the encoding inside the export dialog.