Hmm… maybe this could be incorporated into the site itself (not a "www" link) and therefore the invitation page could be edited by the site members to be in whatever language you wish.

Similar to how we can now add custom error messages for private categories, via the _public page.

Rating + 1 anyway. I'd never use multilingual features myself unless it was translating into English (not from), but I agree that it's necessary as some of the users on my site don't have English as their first language (needless to say, they don't post much, because of the language barrier)