One standard is gettext with .po files. It's widely used and supports lots of things like pluralization rules. This StackOverflow thread suggests two Javascript implementations you might consider. There are PHP libraries as well for sever-side. This post discusses differences with iOS.
edit: try http://zanata.org/
Here is one translation facilitation software option I found that I know nothing about: http://zanata.org/features/
It looks like it might be what we need! It looks really cool plus open source so we could add Lojban syntax highlighting (maybe) if we wanted.
I've done work with integrating zanata (http://zanata.org/) and Drupal in the past that worked extremely well. It required some custom integration work (about 2 weeks), but that might have been more to do with running domain access and drupal commerce in Drupal 7 together.
Alternatively there's lingotek as a paid 3rd party solution. I don't have any direct experience with them but have heard from others they are priced almost exclusively at the enterprise level.