Allow setting of default language through config

Setting of locale in controller extracted to Localized concern,
the doorkeeper authorized applications controller moved under
custom namespace with inclusion of Localized, which resolves the
"it sometimes appears in a different random language" bug

.env.production.sample

@@ -25,6 +25,9 @@ OTP_SECRET=
 # Only allow registrations with the following e-mail domains
 # EMAIL_DOMAIN_WHITELIST=example1.com|example2.de|etc
 
+# Optionally change default language
+# DEFAULT_LOCALE=de
+
 # E-mail configuration
 SMTP_SERVER=smtp.mailgun.org
 SMTP_PORT=587

app/controllers/application_controller.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class ApplicationController < ActionController::Base
+  include Localized
+
   # Prevent CSRF attacks by raising an exception.
   # For APIs, you may want to use :null_session instead.
   protect_from_forgery with: :exception
@@ -14,7 +16,6 @@ class ApplicationController < ActionController::Base
   rescue_from ActionController::InvalidAuthenticityToken, with: :unprocessable_entity
 
   before_action :store_current_location, except: :raise_not_found, unless: :devise_controller?
-  before_action :set_locale
   before_action :set_user_activity
   before_action :check_suspension, if: :user_signed_in?
 
@@ -28,12 +29,6 @@ class ApplicationController < ActionController::Base
     store_location_for(:user, request.url)
   end
 
-  def set_locale
-    I18n.locale = current_user.try(:locale) || I18n.default_locale
-  rescue I18n::InvalidLocale
-    I18n.locale = I18n.default_locale
-  end
-
   def require_admin!
     redirect_to root_path unless current_user&.admin?
   end

app/controllers/concerns/localized.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module Localized
+  extend ActiveSupport::Concern
+
+  included do
+    before_action :set_locale
+  end
+
+  def set_locale
+    I18n.locale = current_user.try(:locale) || default_locale
+  rescue I18n::InvalidLocale
+    I18n.locale = default_locale
+  end
+
+  def default_locale
+    ENV.fetch('DEFAULT_LOCALE') { I18n.default_locale }
+  end
+end

app/controllers/oauth/authorizations_controller.rb

@@ -1,9 +1,10 @@
 # frozen_string_literal: true
 
 class Oauth::AuthorizationsController < Doorkeeper::AuthorizationsController
+  include Localized
+
   skip_before_action :authenticate_resource_owner!
 
-  before_action :set_locale
   before_action :store_current_location
   before_action :authenticate_resource_owner!
 
@@ -12,10 +13,4 @@ class Oauth::AuthorizationsController < Doorkeeper::AuthorizationsController
   def store_current_location
     store_location_for(:user, request.url)
   end
-
-  def set_locale
-    I18n.locale = current_user.try(:locale) || I18n.default_locale
-  rescue I18n::InvalidLocale
-    I18n.locale = I18n.default_locale
-  end
 end

app/controllers/oauth/authorized_applications_controller.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class Oauth::AuthorizedApplicationsController < Doorkeeper::AuthorizedApplicationsController
+  include Localized
+
+  skip_before_action :authenticate_resource_owner!
+
+  before_action :store_current_location
+  before_action :authenticate_resource_owner!
+
+  private
+
+  def store_current_location
+    store_location_for(:user, request.url)
+  end
+end

config/routes.rb

@@ -11,7 +11,7 @@ Rails.application.routes.draw do
   end
 
   use_doorkeeper do
-    controllers authorizations: 'oauth/authorizations'
+    controllers authorizations: 'oauth/authorizations', authorized_applications: 'oauth/authorized_applications'
   end
 
   get '.well-known/host-meta', to: 'xrd#host_meta', as: :host_meta