Locale files get precompiled after deployment and they contained translations from the `default_locale`. That's especially bad in multisites, because the initial `default_locale` is `en_US`. Sites where the `default_locale` isn't `en_US` could see missing translations. The same thing could happen when users are allowed to chose a different locale.
This change simplifies the logic by not using the `default_locale` in the locale chain. It always falls back to `en` in case of missing translations.
Gerhard Schlager