#application.rb
config.i18n.default_locale = :nl
config.i18n.available_locales = [:en, :nl]

before_filter :set_locale
def set_locale
 if [RailsAdmin].include?(self.class.parent)
   I18n.locale = :en
 else
   I18n.locale = params[:locale] || I18n.default_locale
 end
end

#/config/initializers/rails_admin.rb
RailsAdmin.config do |config|
  config.parent_controller = '::ApplicationController'
end