new language and custom translations > solutions?

Since I started my shop, I've added quite some custom translations to my main and second language.

Now I'm getting ready to add a few more languages, but am not thrilled by the prospect to have to check each and every language file to make sure everything is translated correctly.

Is there a way to compare the main language to the new one and filter out the translations that are missing?

I would be interested in this too. Maybe this project should start using a Pootle server or Transifex as a platform, it's getting clumsy doing all the translation updates manually.