Specifying Locale

You just need to set I18n.locale in a before_action.

Add the following line to config/initializers/active_admin.rb:

#config/initializers/active_admin.rb
config.before_action :set_admin_locale

or if you are using Rails 5.1+:

#config/initializers/active_admin.rb
config.before_action :set_admin_locale

and define the following method in your application_controller.rb:

def set_admin_locale
  I18n.locale = :en
end

Of course, put whatever logic you want into the set_admin_locale method. If your AdminUser class has a locale field to store the user's locale, you can do:

def set_admin_locale
  I18n.locale = current_admin_user && current_admin_user.locale || I18n.default_locale
end

Switching Locale

If your use case is to be able to switch locale with language links and persist this locale, here is a way.

You set a default locale in a before filter by adding the following lines to application_controller.rb:

  before_action :set_locale

  def set_locale
    I18n.locale = params[:locale] || I18n.default_locale
  end

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

or follow the Specifying locale guide.

Let's add a menu for selecting languages in active_admin.rb:

 config.namespace :admin do |admin|
    admin.build_menu :utility_navigation do |menu|
      menu.add :label => "Languages" do |lang|
        lang.add :label => "English",:url => proc { url_for(:locale => 'en') }, id: 'i18n-en', :priority => 1
        lang.add :label => "Spanish",:url => proc { url_for(:locale => 'es') }, id: 'i18n-es', :priority => 2
      end
      menu.add :label => proc { display_name current_active_admin_user },
                :url => '#',
                :id => 'current_user',
                :if => proc { current_active_admin_user? }
      admin.add_logout_button_to_menu menu
    end
  end

Prefix url with the locale in routes.rb:

  scope ":locale", :path_prefix => '/:locale' do
    ActiveAdmin.routes(self)
  end

Edit: For a rails 4, use this instead:

scope ':locale', defaults: { locale: I18n.locale } do
  ActiveAdmin.routes(self)
end 

You now have an active admin app that persists locale between queries and permits the user to choose his locale.

You will notice, however, that all links keep the default locale of your app.

You set the default locale in application.rb:

 config.before_configuration do
    I18n.load_path += Dir[Rails.root.join('config', 'locales', '*.{rb,yml}').to_s]
    I18n.default_locale = :en
    I18n.reload!
 end

You can override this default locale by passing the locale to all _path methods.

The example below reproduces the default actions of active admin including cancan authorization and the locale selected by the user. Let's say we have a AA page customers.rb:

ActiveAdmin.register Customer do
  menu :priority => 2, url: ->{ app_customers_path(locale: I18n.locale) } # Pass the locale to the menu link

  action_item do
    link_to I18n.t("New Customer"), new_app_customer_path(locale: I18n.locale) # Pass the locale to the new button
  end

index do
    column :firstname
    column :lastname
    # Reproduce the default actions of AA, authorization with cancan, pass the locale
    column I18n.t("Actions") do |customer|
      link_to(I18n.t("Add Coupon"), new_app_coupon_path(:customer_id => customer.id, :locale => I18n.locale )) + " | " + \
      link_to(I18n.t("Add Order"), new_app_order_path(:customer_id => customer.id, :locale => I18n.locale))
    end

    actions defaults: false do |customer|
      links = ''.html_safe
      if can?(:read, customer)
        links += link_to I18n.t('active_admin.view'), app_customer_path(customer, locale: I18n.locale), class: "member_link view_link"
      end

      if can?(:update, customer)
        links += link_to I18n.t('active_admin.edit'), edit_app_customer_path(customer, locale: I18n.locale), class: "member_link edit_link"
      end

      if can?(:destroy, customer)
        links += link_to I18n.t('active_admin.delete'), app_customer_path(customer, locale: I18n.locale), :method => :delete, :confirm => I18n.t('active_admin.delete_confirmation'), class: "member_link delete_link"
      end
      links
    end
end