We have built a couple of translation plugins before that allows a site visitor to select a language and then have the contents of the site appear in that language. There was a section that allowed a site admin to add translations for all of the different content displayed on the site, it supported several different languages. If you send an email to email@example.com we can give you an estimate to implement the feature.
I'd also have a look at Google embedded website translator system (although the site seems to be down at the moment).:
This system would be much easy to implement, but the translations might be less accurate.
PHP Programmer - interactivetools.com