Problems with :locale in ApplicationController and SubscriptionsMailer

[dannatobigodo]

In a project we use a scope in routes for language switching, which includes the commontator mount:

scope path: '(:locale)', constraints: { locale: /#{I18n.available_locales.join("|")}/ } do
  mount Commontator::Engine => '/commontator'
  # ...
end

We had to customize ApplicationController and SubscriptionMailer due to routing errors in using url helpers:

class Commontator::ApplicationController < ActionController::Base
  before_action :set_locale

  def set_locale
    I18n.locale = params[:locale] || I18n.default_locale
  end

  def url_options(options = {})
    options.merge(locale: I18n.locale, only_path: true)
  end
  # ...
end

class Commontator::SubscriptionsMailer < ActionMailer::Base
  def url_options
    { locale: I18n.locale }.merge(super)
  end
  # ...
end

In particular, Commontator failed to send notifications of new comments, because the :locale parameter was filled with the commontable object, resulting in the error: "No route matches ... missing required keys: [: id]".
Also, after creating a comment, the current :locale was not passed and translations in the default language appeared.