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/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