Inject app name from settings to be available for all translations as an interpolation.

en-US.yml

en-US:
  welcome: "Welcome to %{app_name}!"
  goodbye: "See ya later, %{name}. Come back to %{app_name} soon!"

example.rb

Setting.set(:app_name, "Cool Web App")

I18n.t('welcome')
# => "Welcome to Cool Web App!"

I18n.t('goodbye', name: "Phillip")
# => "See ya later, Phillip. Come back to Cool Web App soon!"

i18n.rb

# Override I18n::translate to pull in app_name from settings
module I18nAppNameInjector
  def translate(*args)
    options = args.extract_options!
    options.reverse_merge!({ app_name: Setting.get(:app_name) })

    super(*args, options)
  end

  alias_method :t, :translate
end

# Ruby 2.0 Module#prepend for the win!
module I18n
  class << self
    prepend I18nAppNameInjector
  end
end