For some time, we use Pootle as online translation tool for phpMyAdmin. It is indeed great tool, but somehow we seem to be hitting more and more issues with it and I feel it really won't fit nicely once we will want to support translations on two divergent branches. Also it's code base is not something easy to understand so that I would be able to fix bugs myself without major effort.
So this is when I started to look for alternatives. Unfortunately there is nothing I could find what would work in way I want. I've looked at Transifex, Translatewiki, Crowdin and others whose names I've forgotten and I still haven't found what I'm looking for.
The key feature I want from such system is full integration with git. This means that whatever is changed should be committed to local git branch and merging translations back should be as easy as merging remote branch. I've hacked such thing into Pootle (in quite dirty way), so this is basically workflow we have right now and I'm pretty much happy with that.
Another thing (which seems to be existing in some tools) is support for simultaneous translation of more branches - if same string exists in more branches, it should be translated in all of them. And last but not least, it should support import of po files allowing users offline translation.
Thinking about this I'm starting to thing that it should be pretty easy to implement something what I want using existing pieces of software without major effort. I'll probably start to play with Django, translate-tookit and GitPython to see if I can come up with something working reasonably well in few days...
PS: Anybody working on updated GitPython packages for Debian, looks like I will need them soon :-).
PS 2: The solution for me is Weblate.
Maybe you should look at Damned lies as a base for your work.
Check it out at http://l10n.gnome.org/