Rails: Switch locale in descendant of Devise controller

Problem

I have this class-inheritance set up with switching to a Dutch locale (according to https://guides.rubyonrails.org/i18n.html, May 20, 2020):

class Api::DeviseSessionsController < Devise::SessionsController
  around_action :switch_locale
  
  private def switch_locale(&action)
    I18n.with_locale('nl-NL', &action)
  end

But unfortunately some Devise error messages, especially the login errors, are still using the English locale.

Solution

In ApplicationController:

before_action :set_locale

private def set_locale
  if (self.class < ::DeviseController)
    I18n.locale = 'nl-NL'
  end
end

Now I get my error messages in Dutch, e.g. "Ongeldig e-mail adres of wachtwoord" :)

Journal

20200522

Got this error:

ArgumentError (A copy of ApplicationController has been removed
from the module tree but is still active!)

Solution: Prefix Devise constant with double colon (::) to define it as a top-level constant.

If that doesn't work, changing the 'parent_controller' to a MyDeviseController of our own with proper I18n, in config/initializers/devise.rb might be a workaround:

# ==> Controller configuration
# Configure the parent class to the devise controllers.
# config.parent_controller = 'DeviseController'

20200520

Putting this in ApplicationController might also work, but somehow my Vue-application behaves differently and causes yet another, some kind of 404-error:

around_action :switch_locale
 
private def switch_locale(&action)
  if (self.class < DeviseController)
    I18n.with_locale('nl-NL', &action)
  end
end

so I'll stick to the 'before_action' for now.