FYI, Sunderbird's builtin *.cvs export, too. (builtin *.html and *.ics export is OK.)
Good news and bad news, I'm afraid.
The good news is that it the problem is probably just that the exstension exports as ANSI rather than UTF-8, which should be a relatively easy fix. This is catered for in the spec for vCalendar as well. It's the same as the .csv export since I used that as a base to design the exporter originally.
The bad news is that I'm heading off on holiday for the month of October (in a couple of hours, in fact) so I won't be able to do anything about it until November, since I'll be nowhere near the code. Sorry. This also means that if Sunbird 0.7 comes out in the next week or two, this extension wont be made compatible for a few weeks after. Lightning should be OK since compatibility bepends on Thunderbird, not Lightning.