FIX: Using the default_locale in locale fallbacks caused problems

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.

lib/i18n/backend/fallback_locale_list.rb

@@ -6,19 +6,15 @@ module I18n
     class FallbackLocaleList < Hash
       def [](locale)
         locale = locale.to_sym
-        return [locale] if locale == :en
+        locale_list = [locale]
+        return locale_list if locale == :en
 
-        fallback_locale = LocaleSiteSetting.fallback_locale(locale)
-        site_locale = SiteSetting.default_locale.to_sym
-
-        locale_list =
-          if locale == site_locale || site_locale == :en || fallback_locale == :en
-            [locale, fallback_locale, :en]
-          else
-            site_fallback_locale = LocaleSiteSetting.fallback_locale(site_locale)
-            [locale, fallback_locale, site_locale, site_fallback_locale, :en]
-          end
+        while (fallback_locale = LocaleSiteSetting.fallback_locale(locale))
+          locale_list << fallback_locale
+          locale = fallback_locale
+        end
 
+        locale_list << :en
         locale_list.uniq.compact
       end
     end