I did not use rails_admin, but a quick scan of his repo would show it using any language installed in your application. I assume that you installed this in the before_filter file in your .rb application through one of the methods described in the Rails i18n manual. You will need to do this before filter_a little smarter. Perhaps something like:
if self.kind_of? RailsAdmin::ApplicationController I18n.locale = :en else
source share