As an example, the lookup order for German was:
1. override for de
2. override for en
3. value from de
4. value from en
After this change the lookup order is the same as on the client:
1. override for de
2. value from de
3. override for en
4. value from en
see /t/16381
Plugin client.en.yml and server.en.yml can now be client/server-(1-100).en.yml. 1 is the lowest priority, and 100 is the highest priority. This allows plugins to set their priority higher than other plugins, so that they can override eachothers' translations.
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.
