Add i18n locale param support

app/controllers/shopify_app/authenticated_controller.rb

@@ -1,8 +1,12 @@
 module ShopifyApp
   class AuthenticatedController < ApplicationController
+    include ShopifyApp::Localization
     include ShopifyApp::LoginProtection
+
+    before_action :set_locale
     before_action :login_again_if_different_shop
     around_action :shopify_session
+
     layout ShopifyApp.configuration.embedded_app? ? 'embedded_app' : 'application'
   end
 end

lib/shopify_app.rb

@@ -18,6 +18,7 @@
 require 'shopify_app/shop'
 require 'shopify_app/session_storage'
 require 'shopify_app/sessions_concern'
+require 'shopify_app/localization'
 require 'shopify_app/login_protection'
 require 'shopify_app/webhooks_manager'
 require 'shopify_app/scripttags_manager'

lib/shopify_app/localization.rb

@@ -0,0 +1,16 @@
+module ShopifyApp
+  module Localization
+    extend ActiveSupport::Concern
+
+    def set_locale
+      if params[:locale]
+        session[:locale] = params[:locale]
+      else
+        session[:locale] ||= I18n.default_locale
+      end
+      I18n.locale = session[:locale]
+    rescue I18n::InvalidLocale
+      I18n.locale = I18n.default_locale
+    end
+  end
+end

test/shopify_app/localization_test.rb

@@ -0,0 +1,57 @@
+require 'test_helper'
+require 'action_controller'
+require 'action_controller/base'
+
+class LocalizationController < ActionController::Base
+  include ShopifyApp::Localization
+
+  before_action :set_locale
+
+  def index
+    render text: I18n.locale
+  end
+end
+
+class LocalizationTest < ActionController::TestCase
+  tests LocalizationController
+
+  test "falls back to I18n.default if locale param is not present" do
+    I18n.available_locales = [:en, :de, :es, :ja, :fr]
+    I18n.default_locale = :ja
+
+    with_test_routes do
+      get :index
+      assert 'ja', response.body
+    end
+  end
+
+  test "set I18n.locale to passed locale param" do
+    I18n.available_locales = [:en, :de, :es, :ja, :fr]
+
+    with_test_routes do
+      get :index, locale: 'de'
+      assert 'de', response.body
+    end
+  end
+
+  test "falls back to I18n.default if locale is not supported" do
+    I18n.available_locales = [:en, :de, :es, :ja, :fr]
+    I18n.default_locale = :en
+
+    with_test_routes do
+      get :index, locale: 'cu'
+      assert 'en', response.body
+    end
+  end
+
+  private
+
+  def with_test_routes
+    with_routing do |set|
+      set.draw do
+        get '/locale' => 'localization#index'
+      end
+      yield
+    end
+  end
+end