Fix i18n fallback configuration conflicts with environment configurations (#6843)

config/application.rb

@@ -76,9 +76,7 @@ module Mastodon
     ]
 
     config.i18n.default_locale = ENV['DEFAULT_LOCALE']&.to_sym
-    if config.i18n.available_locales.include?(config.i18n.default_locale)
-      config.i18n.fallbacks = [:en]
-    else
+    unless config.i18n.available_locales.include?(config.i18n.default_locale)
       config.i18n.default_locale = :en
     end
 

config/environments/production.rb

@@ -55,8 +55,8 @@ Rails.application.configure do
   # config.action_mailer.raise_delivery_errors = false
 
   # Enable locale fallbacks for I18n (makes lookups for any locale fall back to
-  # the I18n.default_locale when a translation cannot be found).
-  config.i18n.fallbacks = true
+  # English when a translation cannot be found).
+  config.i18n.fallbacks = [:en]
 
   # Send deprecation notices to registered listeners.
   config.active_support.deprecation = :notify