Example application to show issue with using devise and route_translator in the same rails project.

Steps to recreate app

Here are all the steps to recreate this example application from scratch, assuming Ruby and Rails are installed:

Creating a new Rails application

$ rails new drt-example --minimal
$ cd drt-example
$ rails db:create

Add a basic controller

This step is optional, but useful to verify that the route translation setup is working properly.

Create a new controller:

$ rails g controller home index example

Set up the views with some content:

app/views/home/index.html.erb

<h1><%= t(:greetings) %></h1>
<%= link_to t(:example), action: :example %>

app/views/home/example.html.erb

<%= I18n.locale %>

Set up localized routing

Add route_translator to the Gemfile.

Set up localized routes in config/routes.rb:

Rails.application.routes.draw do
  localized do
    root to: 'home#index'
    get 'example', to: 'home#example'
  end
end

Set up locales:

config/locales/en.yml

en:
  greetings: Welcome
  example: Example

config/locales/de.yml

de:
  greetings: Willkommen
  example: Beispiel
  routes:
    example: beispiel

Make sure locale is set for controller/view in app/controllers/application_controller.rb:

class ApplicationController < ActionController::Base
  around_action :set_locale_from_url
end

At this point, everything works as expected.

Set up Devise

Add devise to the Gemfile.

Set up a user class with devise.

$ rails generate devise:install
      create  config/initializers/devise.rb
      create  config/locales/devise.en.yml
$ rails generate devise user
      invoke  active_record
      create    db/migrate/20210206133737_devise_create_users.rb
      create    app/models/user.rb
      invoke    test_unit
      create      test/models/user_test.rb
      create      test/fixtures/users.yml
      insert    app/models/user.rb
       route  devise_for :users
$ rails db:migrate
== 20210206133737 DeviseCreateUsers: migrating ================================
-- create_table(:users)
   -> 0.0018s
-- add_index(:users, :email, {:unique=>true})
   -> 0.0006s
-- add_index(:users, :reset_password_token, {:unique=>true})
   -> 0.0006s
== 20210206133737 DeviseCreateUsers: migrated (0.0031s) =======================

Add before_action :authenticate_user! to app/controllers/application_controller.rb. Move the devise routes inside the localized block in config/routes.rb (this is the step that breaks things).

The Issue

Accessing /de without being logged in should redirect to /de/users/sign_in, but redirects to /users/sign_in.

This will result in the login for being in the wrong locale. Moreover, in more complex situations, like host based routing, this will break completely (most likely showing a 404 or 500 error page instead of the log in form).