I’m planning to introduce support for translation in the next update. So far, I’ve got interest in French and Danish. I’m a mono-linguistic American, so my contribution will be limited to tagging words and phrases for translation and looking for collaborators who can do the actual translating.
The process is outlined at http://codex.wordpress.org/I18n_for_WordPress_Developers
Update 12/29/2010: I’ve finished flagging most of the elements in the user interface that I thought might need to be translated and have added them to the file rsvp.pot in the plugins/rsvpmaker/translations subdirectory of the latest release. This is the “catalog” file that you load into a tool such as POEdit to translate words and phrases such as “RSVP Now!” into whatever makes more sense in a given language.
Send me your translations to be included with a future release, and I will be happy to credit your contribution.