It makes the localisation at translatewiki.net tricky when the goal is to get 100% localisation in many, many languages. Full localisation is however a great motivator and we are again back to four languages in our "shooting on a moving target" challenge.
Ryan Kaldari added two messages in r76614 and only one needs localisation. They are:
month' => '$1 ($2)',
month' => 'Expiration month for credit card. $1 is the month number, $2 is the month name.',