# config/application.rb
  class Application < Rails::Application
    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration should go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded.
    config.i18n.available_locales = [:en, :ja]
    config.i18n.default_locale = :en
  end

# routes.rb
  scope "(:locale)", locale: /#{I18n.available_locales.join("|")}/ do
    resources :users
    root 'visitors#index'
  end

# application_controller.rb
class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  before_action :set_locale

  private

  def set_locale
    I18n.locale = params[:locale] || I18n.default_locale
    # current_user.locale
    # request.env["HTTP_ACCEPT_LANGUAGE"]
  end

  def default_url_options(options = {})
    { locale: I18n.locale }.merge options
  end
end

# config/locales/en.yml
---
en:
  activerecord:
    attributes:
      user:
        email: Email@
        first_name: First@Name
        last_name: Last@Name
    model:
      user: Users
  back: Back
  date:
    formats:
      default: "%m/%d/%Y"
      short: "%m/%d"
  edit: Edit
  greetings: Hello guest!
  visitors:
    index:
      greetings: Hello guest!

# _navigation_links.html.erb
<%= content_tag :li do %>
  <%= link_to User.model_name.human, users_path %>
<% end %>

<%= content_tag :li do %>
  <%= link_to 'Switch to English', locale: :en %>
<% end unless current_page?(locale: :en) %>

<%= content_tag :li do %>
  <%= link_to 'Switch to Japanese', locale: :ja %>
<% end unless current_page?(locale: :ja) %>

# different translations used
<%= t '.greetings' %>
<%= l Date.today %
<%= l Date.today, format: :short %>
<%= User.model_name.human %>
<%= User.human_attribute_name(:first_name) %>
<%= User.human_attribute_name(:last_name) %>
<%= User.human_attribute_name(:email) %>
<%= link_to t(:edit), edit_user_path(@user) %>
<%= link_to t(:back), users_path %>

# i18n-tasks
gem install i18n-tasks

# common commands
i18n-tasks health | missing | unused
i18n-tasks add-missing
i18n-tasks normalize

# sample output

Added 7 keys
+--------+-----------------------------------------+--------------+
| Locale | Key                                     | Value        |
+--------+-----------------------------------------+--------------+
|   ja   | activerecord.attributes.user.email      | Email@       |
|   ja   | activerecord.attributes.user.first_name | First@Name   |
|   ja   | activerecord.attributes.user.last_name  | Last@Name    |
|   ja   | activerecord.model.user                 | Users!       |
|   ja   | back                                    | Back         |
|   ja   | edit                                    | Edit         |
|   ja   | visitors.index.greetings                | Hello guest! |
+--------+-----------------------------------------+--------------+