I don't envy having to work on something of that scale! At least for my particular case, the only item in that list I needed to worry about was plurals (which I have with the __n method). (And thankfully there are other good JS libraries for currency and time/date formatting that I can use.) I recommended people look at Jed.js and messageformat.js for some of the more complex cases, thankfully I haven't had to go down that road yet!
One thing that has always bitten me, is stuff like:
Go to the <a href=/settings>account settings</a> page to change your username.
It is tricky, because you can't just extract three strings, and if you extract the full text, then you have urls and html in the translation files. I have done stuff like ||link|| and ||end_link||, which works okay, but you also need a tool to make sure the translations have matching components.
