I suspect this topic/project would benefit from 3 lists in the first post.
1: Translations already done
2: Translations in progress
3: Translations not yet assigned a translator
I really like your suggestion and we're looking into getting the 'percentage completed' dynamically back to the language download page
(it had been there for a short while). However I do feel translations are never really done. We can always use more people looking at the languages to check if there's something that still needs quality improvement.
And not only that, we also need more translators in order to keep up with the new translatable material that's being added regularly. Next to the updates to the forum software itself, we are planning to add the Simple Machines site content and the documentation as translatable content too.
So, the more translators (regardless of the language) are active, the faster things get translated and the higher the quality of the translations will be.