I assumed that if they could speak at Congress, they could speak English. A video hookup would be easy enough. If they don't speak English I don't think it would be of much use.
Could be worthwhile. We haven't discussed a keynote speaker yet, I just posted about it due to @LMK 's offer. If I understand correctly there isn't a great deal of new policy or Constitutional reforms to be put to Congress, so we will have spare time for discussion.
Having multiple speakers on participatory democracy would give Congress a theme, which could be good. That said, I personally would rather have time set aside to talk about party strategy than giving a lot of time to participatory democracy as it is a higher priority.