Fix Rubocop complains plus some small refactor

app/controllers/application_controller.rb

@@ -269,6 +269,7 @@ class ApplicationController < ActionController::Base
   def set_locale
     requested_locale = current_user&.preferred_language || request.env['HTTP_ACCEPT_LANGUAGE'] || I18n.default_locale
     locale = FastGettext.set_locale(requested_locale)
+
     I18n.locale = locale
   end
 end

app/views/profiles/show.html.haml

@@ -74,7 +74,7 @@
         %span.help-block This email will be displayed on your public profile.
       .form-group
         = f.label :preferred_language, class: "label-light"
-        = f.select :preferred_language, Gitlab::I18n::AVAILABLE_LANGUAGES.map { |lang| [_(lang[0]), lang[1]] },
+        = f.select :preferred_language, Gitlab::I18n::AVAILABLE_LANGUAGES.map { |value, label| [_(label), value] },
           {}, class: "select2"
       .form-group
         = f.label :skype, class: "label-light"

config/initializers/fast_gettext.rb

 FastGettext.add_text_domain 'gitlab', path: 'locale', type: :po
-FastGettext.default_available_locales = ['en', 'es','de']
+FastGettext.default_available_locales = Gitlab::I18n::AVAILABLE_LANGUAGES.keys
 FastGettext.default_text_domain = 'gitlab'

db/migrate/20170413035209_add_preferred_language_to_users.rb

@@ -8,7 +8,11 @@ class AddPreferredLanguageToUsers < ActiveRecord::Migration
 
   disable_ddl_transaction!
 
-  def change
+  def up
     add_column_with_default :users, :preferred_language, :string, default: 'en'
   end
+
+  def down
+    remove_column :users, :preferred_language
+  end
 end

lib/gitlab/i18n.rb

 module Gitlab
   module I18n
-    AVAILABLE_LANGUAGES = [
-      ['English', 'en'],
-      ['Spanish', 'es'],
-      ['Deutsch', 'de']
-    ]
+    AVAILABLE_LANGUAGES = {
+      'en' => 'English',
+      'es' => 'Spanish',
+      'de' => 'Deutsch'
+    }.freeze
   end
 end