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

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