When building a multilingual system (CMS, framework, website, etc.) make sure your first language is "dev". Everytime you change an origin string every translation has to change that string. That also means, when you change a typo in the original, every translation has to update their strings even if the translation did not include the typo.
By English being the lingua franca of our times (and our dev communities),
en_US usually also is the primary language of any application.