Did you know that Drupal only learns how many plural variants a language has from imported .po files? It has a very fast lookup table to pick the right variant each time, but there is no UI to set or fix the rules. Get them wrong and your carefully translated strings fall back to the wrong form. Export your translations and the right plural rules don't come along either. I had a small module that provided a little bit of the solution since 2010 but recently decided to think bigger.