Here's a few thoughts (without real numbers behind them)
Lots of folks are now using multi-lingual join pages, which is a great step in the right direction, but without multi-lingual tour pages, they're not really presenting a compelling or understandable reason to join and thus the non-English (or whatever language the site is primarily in) speaking prospect won't likely make it to the join page.
Tours and join pages aside, what happens once the surfer enters the member's area?
If he can't readily understand the navigational options, the user experience is diminished and retention suffers as a result, making multi-lingual an approach that must be executed site-wide to maximize effectiveness.
It's easy enough to use geo-IP and other technologies (or simply user selections on cute little flags) to then dynamically generate nav links, but I'm making things a bit easier by going cross-lingual, using English links but also having easily understandable iconic images (such as a still camera image for a link to the photo galleries and a video camera for the link to the videos, etc.) so that regardless of the user's language, he or she can still navigate and enjoy the site.
One other observation: a "Babel Fish" translation may or may not be better than nothing, but it does make a great base from which a professional translator can "clean up" ? something that might make your resulting text more along the lines you intended, while also saving you a few pesos.
